analyzejoins.c

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/*-------------------------------------------------------------------------
 *
 * analyzejoins.c
 *    Routines for simplifying joins after initial query analysis
 *
 * While we do a great deal of join simplification in prep/prepjointree.c,
 * certain optimizations cannot be performed at that stage for lack of
 * detailed information about the query.  The routines here are invoked
 * after initsplan.c has done its work, and can do additional join removal
 * and simplification steps based on the information extracted.  The penalty
 * is that we have to work harder to clean up after ourselves when we modify
 * the query, since the derived data structures have to be updated too.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/optimizer/plan/analyzejoins.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "catalog/pg_class.h"
#include "nodes/nodeFuncs.h"
#include "optimizer/joininfo.h"
#include "optimizer/optimizer.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/placeholder.h"
#include "optimizer/planmain.h"
#include "optimizer/restrictinfo.h"
#include "rewrite/rewriteManip.h"
#include "utils/lsyscache.h"
 
/*
 * Utility structure.  A sorting procedure is needed to simplify the search
 * of SJE-candidate baserels referencing the same database relation.  Having
 * collected all baserels from the query jointree, the planner sorts them
 * according to the reloid value, groups them with the next pass and attempts
 * to remove self-joins.
 *
 * Preliminary sorting prevents quadratic behavior that can be harmful in the
 * case of numerous joins.
 */
typedef struct
{
    int         relid;
    Oid         reloid;
} SelfJoinCandidate;
 
bool        enable_self_join_elimination;
 
/* local functions */
static bool join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo);
static void remove_leftjoinrel_from_query(PlannerInfo *root, int relid,
                                          SpecialJoinInfo *sjinfo);
static void remove_rel_from_restrictinfo(RestrictInfo *rinfo,
                                         int relid, int ojrelid);
static void remove_rel_from_eclass(EquivalenceClass *ec,
                                   SpecialJoinInfo *sjinfo,
                                   int relid, int subst);
static List *remove_rel_from_joinlist(List *joinlist, int relid, int *nremoved);
static bool rel_supports_distinctness(PlannerInfo *root, RelOptInfo *rel);
static bool rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel,
                                List *clause_list, List **extra_clauses);
static Oid  distinct_col_search(int colno, List *colnos, List *opids);
static bool is_innerrel_unique_for(PlannerInfo *root,
                                   Relids joinrelids,
                                   Relids outerrelids,
                                   RelOptInfo *innerrel,
                                   JoinType jointype,
                                   List *restrictlist,
                                   List **extra_clauses);
static int  self_join_candidates_cmp(const void *a, const void *b);
static bool replace_relid_callback(Node *node,
                                   ChangeVarNodes_context *context);
 
 
/*
 * remove_useless_joins
 *      Check for relations that don't actually need to be joined at all,
 *      and remove them from the query.
 *
 * We are passed the current joinlist and return the updated list.  Other
 * data structures that have to be updated are accessible via "root".
 */
List *
remove_useless_joins(PlannerInfo *root, List *joinlist)
{
    ListCell   *lc;
 
    /*
     * We are only interested in relations that are left-joined to, so we can
     * scan the join_info_list to find them easily.
     */
restart:
    foreach(lc, root->join_info_list)
    {
        SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(lc);
        int         innerrelid;
        int         nremoved;
 
        /* Skip if not removable */
        if (!join_is_removable(root, sjinfo))
            continue;
 
        /*
         * Currently, join_is_removable can only succeed when the sjinfo's
         * righthand is a single baserel.  Remove that rel from the query and
         * joinlist.
         */
        innerrelid = bms_singleton_member(sjinfo->min_righthand);
 
        remove_leftjoinrel_from_query(root, innerrelid, sjinfo);
 
        /* We verify that exactly one reference gets removed from joinlist */
        nremoved = 0;
        joinlist = remove_rel_from_joinlist(joinlist, innerrelid, &nremoved);
        if (nremoved != 1)
            elog(ERROR, "failed to find relation %d in joinlist", innerrelid);
 
        /*
         * We can delete this SpecialJoinInfo from the list too, since it's no
         * longer of interest.  (Since we'll restart the foreach loop
         * immediately, we don't bother with foreach_delete_current.)
         */
        root->join_info_list = list_delete_cell(root->join_info_list, lc);
 
        /*
         * Restart the scan.  This is necessary to ensure we find all
         * removable joins independently of ordering of the join_info_list
         * (note that removal of attr_needed bits may make a join appear
         * removable that did not before).
         */
        goto restart;
    }
 
    return joinlist;
}
 
/*
 * join_is_removable
 *    Check whether we need not perform this special join at all, because
 *    it will just duplicate its left input.
 *
 * This is true for a left join for which the join condition cannot match
 * more than one inner-side row.  (There are other possibly interesting
 * cases, but we don't have the infrastructure to prove them.)  We also
 * have to check that the inner side doesn't generate any variables needed
 * above the join.
 */
static bool
join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
{
    int         innerrelid;
    RelOptInfo *innerrel;
    Relids      inputrelids;
    Relids      joinrelids;
    List       *clause_list = NIL;
    ListCell   *l;
    int         attroff;
 
    /*
     * Must be a left join to a single baserel, else we aren't going to be
     * able to do anything with it.
     */
    if (sjinfo->jointype != JOIN_LEFT)
        return false;
 
    if (!bms_get_singleton_member(sjinfo->min_righthand, &innerrelid))
        return false;
 
    /*
     * Never try to eliminate a left join to the query result rel.  Although
     * the case is syntactically impossible in standard SQL, MERGE will build
     * a join tree that looks exactly like that.
     */
    if (innerrelid == root->parse->resultRelation)
        return false;
 
    innerrel = find_base_rel(root, innerrelid);
 
    /*
     * Before we go to the effort of checking whether any innerrel variables
     * are needed above the join, make a quick check to eliminate cases in
     * which we will surely be unable to prove uniqueness of the innerrel.
     */
    if (!rel_supports_distinctness(root, innerrel))
        return false;
 
    /* Compute the relid set for the join we are considering */
    inputrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);
    Assert(sjinfo->ojrelid != 0);
    joinrelids = bms_copy(inputrelids);
    joinrelids = bms_add_member(joinrelids, sjinfo->ojrelid);
 
    /*
     * We can't remove the join if any inner-rel attributes are used above the
     * join.  Here, "above" the join includes pushed-down conditions, so we
     * should reject if attr_needed includes the OJ's own relid; therefore,
     * compare to inputrelids not joinrelids.
     *
     * As a micro-optimization, it seems better to start with max_attr and
     * count down rather than starting with min_attr and counting up, on the
     * theory that the system attributes are somewhat less likely to be wanted
     * and should be tested last.
     */
    for (attroff = innerrel->max_attr - innerrel->min_attr;
         attroff >= 0;
         attroff--)
    {
        if (!bms_is_subset(innerrel->attr_needed[attroff], inputrelids))
            return false;
    }
 
    /*
     * Similarly check that the inner rel isn't needed by any PlaceHolderVars
     * that will be used above the join.  The PHV case is a little bit more
     * complicated, because PHVs may have been assigned a ph_eval_at location
     * that includes the innerrel, yet their contained expression might not
     * actually reference the innerrel (it could be just a constant, for
     * instance).  If such a PHV is due to be evaluated above the join then it
     * needn't prevent join removal.
     */
    foreach(l, root->placeholder_list)
    {
        PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(l);
 
        if (bms_overlap(phinfo->ph_lateral, innerrel->relids))
            return false;       /* it references innerrel laterally */
        if (!bms_overlap(phinfo->ph_eval_at, innerrel->relids))
            continue;           /* it definitely doesn't reference innerrel */
        if (bms_is_subset(phinfo->ph_needed, inputrelids))
            continue;           /* PHV is not used above the join */
        if (!bms_is_member(sjinfo->ojrelid, phinfo->ph_eval_at))
            return false;       /* it has to be evaluated below the join */
 
        /*
         * We need to be sure there will still be a place to evaluate the PHV
         * if we remove the join, ie that ph_eval_at wouldn't become empty.
         */
        if (!bms_overlap(sjinfo->min_lefthand, phinfo->ph_eval_at))
            return false;       /* there isn't any other place to eval PHV */
        /* Check contained expression last, since this is a bit expensive */
        if (bms_overlap(pull_varnos(root, (Node *) phinfo->ph_var->phexpr),
                        innerrel->relids))
            return false;       /* contained expression references innerrel */
    }
 
    /*
     * Search for mergejoinable clauses that constrain the inner rel against
     * either the outer rel or a pseudoconstant.  If an operator is
     * mergejoinable then it behaves like equality for some btree opclass, so
     * it's what we want.  The mergejoinability test also eliminates clauses
     * containing volatile functions, which we couldn't depend on.
     */
    foreach(l, innerrel->joininfo)
    {
        RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(l);
 
        /*
         * If the current join commutes with some other outer join(s) via
         * outer join identity 3, there will be multiple clones of its join
         * clauses in the joininfo list.  We want to consider only the
         * has_clone form of such clauses.  Processing more than one form
         * would be wasteful, and also some of the others would confuse the
         * RINFO_IS_PUSHED_DOWN test below.
         */
        if (restrictinfo->is_clone)
            continue;           /* ignore it */
 
        /*
         * If it's not a join clause for this outer join, we can't use it.
         * Note that if the clause is pushed-down, then it is logically from
         * above the outer join, even if it references no other rels (it might
         * be from WHERE, for example).
         */
        if (RINFO_IS_PUSHED_DOWN(restrictinfo, joinrelids))
            continue;           /* ignore; not useful here */
 
        /* Ignore if it's not a mergejoinable clause */
        if (!restrictinfo->can_join ||
            restrictinfo->mergeopfamilies == NIL)
            continue;           /* not mergejoinable */
 
        /*
         * Check if the clause has the form "outer op inner" or "inner op
         * outer", and if so mark which side is inner.
         */
        if (!clause_sides_match_join(restrictinfo, sjinfo->min_lefthand,
                                     innerrel->relids))
            continue;           /* no good for these input relations */
 
        /* OK, add to list */
        clause_list = lappend(clause_list, restrictinfo);
    }
 
    /*
     * Now that we have the relevant equality join clauses, try to prove the
     * innerrel distinct.
     */
    if (rel_is_distinct_for(root, innerrel, clause_list, NULL))
        return true;
 
    /*
     * Some day it would be nice to check for other methods of establishing
     * distinctness.
     */
    return false;
}
 
 
/*
 * Remove the target rel->relid and references to the target join from the
 * planner's data structures, having determined that there is no need
 * to include them in the query. Optionally replace them with subst if subst
 * is non-negative.
 *
 * This function updates only parts needed for both left-join removal and
 * self-join removal.
 */
static void
remove_rel_from_query(PlannerInfo *root, RelOptInfo *rel,
                      int subst, SpecialJoinInfo *sjinfo,
                      Relids joinrelids)
{
    int         relid = rel->relid;
    Index       rti;
    ListCell   *l;
 
    /*
     * Update all_baserels and related relid sets.
     */
    root->all_baserels = adjust_relid_set(root->all_baserels, relid, subst);
    root->all_query_rels = adjust_relid_set(root->all_query_rels, relid, subst);
 
    if (sjinfo != NULL)
    {
        root->outer_join_rels = bms_del_member(root->outer_join_rels,
                                               sjinfo->ojrelid);
        root->all_query_rels = bms_del_member(root->all_query_rels,
                                              sjinfo->ojrelid);
    }
 
    /*
     * Likewise remove references from SpecialJoinInfo data structures.
     *
     * This is relevant in case the outer join we're deleting is nested inside
     * other outer joins: the upper joins' relid sets have to be adjusted. The
     * RHS of the target outer join will be made empty here, but that's OK
     * since caller will delete that SpecialJoinInfo entirely.
     */
    foreach(l, root->join_info_list)
    {
        SpecialJoinInfo *sjinf = (SpecialJoinInfo *) lfirst(l);
 
        /*
         * initsplan.c is fairly cavalier about allowing SpecialJoinInfos'
         * lefthand/righthand relid sets to be shared with other data
         * structures.  Ensure that we don't modify the original relid sets.
         * (The commute_xxx sets are always per-SpecialJoinInfo though.)
         */
        sjinf->min_lefthand = bms_copy(sjinf->min_lefthand);
        sjinf->min_righthand = bms_copy(sjinf->min_righthand);
        sjinf->syn_lefthand = bms_copy(sjinf->syn_lefthand);
        sjinf->syn_righthand = bms_copy(sjinf->syn_righthand);
        /* Now remove relid from the sets: */
        sjinf->min_lefthand = adjust_relid_set(sjinf->min_lefthand, relid, subst);
        sjinf->min_righthand = adjust_relid_set(sjinf->min_righthand, relid, subst);
        sjinf->syn_lefthand = adjust_relid_set(sjinf->syn_lefthand, relid, subst);
        sjinf->syn_righthand = adjust_relid_set(sjinf->syn_righthand, relid, subst);
 
        if (sjinfo != NULL)
        {
            Assert(subst <= 0);
 
            /* Remove sjinfo->ojrelid bits from the sets: */
            sjinf->min_lefthand = bms_del_member(sjinf->min_lefthand,
                                                 sjinfo->ojrelid);
            sjinf->min_righthand = bms_del_member(sjinf->min_righthand,
                                                  sjinfo->ojrelid);
            sjinf->syn_lefthand = bms_del_member(sjinf->syn_lefthand,
                                                 sjinfo->ojrelid);
            sjinf->syn_righthand = bms_del_member(sjinf->syn_righthand,
                                                  sjinfo->ojrelid);
            /* relid cannot appear in these fields, but ojrelid can: */
            sjinf->commute_above_l = bms_del_member(sjinf->commute_above_l,
                                                    sjinfo->ojrelid);
            sjinf->commute_above_r = bms_del_member(sjinf->commute_above_r,
                                                    sjinfo->ojrelid);
            sjinf->commute_below_l = bms_del_member(sjinf->commute_below_l,
                                                    sjinfo->ojrelid);
            sjinf->commute_below_r = bms_del_member(sjinf->commute_below_r,
                                                    sjinfo->ojrelid);
        }
        else
        {
            Assert(subst > 0);
 
            ChangeVarNodesExtended((Node *) sjinf->semi_rhs_exprs, relid, subst,
                                   0, replace_relid_callback);
        }
    }
 
    /*
     * Likewise remove references from PlaceHolderVar data structures,
     * removing any no-longer-needed placeholders entirely.  We remove PHV
     * only for left-join removal.  With self-join elimination, PHVs already
     * get moved to the remaining relation, where they might still be needed.
     * It might also happen that we skip the removal of some PHVs that could
     * be removed.  However, the overhead of extra PHVs is small compared to
     * the complexity of analysis needed to remove them.
     *
     * Removal is a bit trickier than it might seem: we can remove PHVs that
     * are used at the target rel and/or in the join qual, but not those that
     * are used at join partner rels or above the join.  It's not that easy to
     * distinguish PHVs used at partner rels from those used in the join qual,
     * since they will both have ph_needed sets that are subsets of
     * joinrelids.  However, a PHV used at a partner rel could not have the
     * target rel in ph_eval_at, so we check that while deciding whether to
     * remove or just update the PHV.  There is no corresponding test in
     * join_is_removable because it doesn't need to distinguish those cases.
     */
    foreach(l, root->placeholder_list)
    {
        PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(l);
 
        Assert(sjinfo == NULL || !bms_is_member(relid, phinfo->ph_lateral));
        if (sjinfo != NULL &&
            bms_is_subset(phinfo->ph_needed, joinrelids) &&
            bms_is_member(relid, phinfo->ph_eval_at) &&
            !bms_is_member(sjinfo->ojrelid, phinfo->ph_eval_at))
        {
            /*
             * This code shouldn't be executed if one relation is substituted
             * with another: in this case, the placeholder may be employed in
             * a filter inside the scan node the SJE removes.
             */
            root->placeholder_list = foreach_delete_current(root->placeholder_list,
                                                            l);
            root->placeholder_array[phinfo->phid] = NULL;
        }
        else
        {
            PlaceHolderVar *phv = phinfo->ph_var;
 
            phinfo->ph_eval_at = adjust_relid_set(phinfo->ph_eval_at, relid, subst);
            if (sjinfo != NULL)
                phinfo->ph_eval_at = adjust_relid_set(phinfo->ph_eval_at,
                                                      sjinfo->ojrelid, subst);
            Assert(!bms_is_empty(phinfo->ph_eval_at));  /* checked previously */
            /* Reduce ph_needed to contain only "relation 0"; see below */
            if (bms_is_member(0, phinfo->ph_needed))
                phinfo->ph_needed = bms_make_singleton(0);
            else
                phinfo->ph_needed = NULL;
 
            phinfo->ph_lateral = adjust_relid_set(phinfo->ph_lateral, relid, subst);
 
            /*
             * ph_lateral might contain rels mentioned in ph_eval_at after the
             * replacement, remove them.
             */
            phinfo->ph_lateral = bms_difference(phinfo->ph_lateral, phinfo->ph_eval_at);
            /* ph_lateral might or might not be empty */
 
            phv->phrels = adjust_relid_set(phv->phrels, relid, subst);
            if (sjinfo != NULL)
                phv->phrels = adjust_relid_set(phv->phrels,
                                               sjinfo->ojrelid, subst);
            Assert(!bms_is_empty(phv->phrels));
 
            ChangeVarNodesExtended((Node *) phv->phexpr, relid, subst, 0,
                                   replace_relid_callback);
 
            Assert(phv->phnullingrels == NULL); /* no need to adjust */
        }
    }
 
    /*
     * Likewise remove references from EquivalenceClasses.
     */
    foreach(l, root->eq_classes)
    {
        EquivalenceClass *ec = (EquivalenceClass *) lfirst(l);
 
        if (bms_is_member(relid, ec->ec_relids) ||
            (sjinfo == NULL || bms_is_member(sjinfo->ojrelid, ec->ec_relids)))
            remove_rel_from_eclass(ec, sjinfo, relid, subst);
    }
 
    /*
     * Finally, we must recompute per-Var attr_needed and per-PlaceHolderVar
     * ph_needed relid sets.  These have to be known accurately, else we may
     * fail to remove other now-removable outer joins.  And our removal of the
     * join clause(s) for this outer join may mean that Vars that were
     * formerly needed no longer are.  So we have to do this honestly by
     * repeating the construction of those relid sets.  We can cheat to one
     * small extent: we can avoid re-examining the targetlist and HAVING qual
     * by preserving "relation 0" bits from the existing relid sets.  This is
     * safe because we'd never remove such references.
     *
     * So, start by removing all other bits from attr_needed sets and
     * lateral_vars lists.  (We already did this above for ph_needed.)
     */
    for (rti = 1; rti < root->simple_rel_array_size; rti++)
    {
        RelOptInfo *otherrel = root->simple_rel_array[rti];
        int         attroff;
 
        /* there may be empty slots corresponding to non-baserel RTEs */
        if (otherrel == NULL)
            continue;
 
        Assert(otherrel->relid == rti); /* sanity check on array */
 
        for (attroff = otherrel->max_attr - otherrel->min_attr;
             attroff >= 0;
             attroff--)
        {
            if (bms_is_member(0, otherrel->attr_needed[attroff]))
                otherrel->attr_needed[attroff] = bms_make_singleton(0);
            else
                otherrel->attr_needed[attroff] = NULL;
        }
 
        if (subst > 0)
            ChangeVarNodesExtended((Node *) otherrel->lateral_vars, relid,
                                   subst, 0, replace_relid_callback);
    }
}
 
/*
 * Remove the target relid and references to the target join from the
 * planner's data structures, having determined that there is no need
 * to include them in the query.
 *
 * We are not terribly thorough here.  We only bother to update parts of
 * the planner's data structures that will actually be consulted later.
 */
static void
remove_leftjoinrel_from_query(PlannerInfo *root, int relid,
                              SpecialJoinInfo *sjinfo)
{
    RelOptInfo *rel = find_base_rel(root, relid);
    int         ojrelid = sjinfo->ojrelid;
    Relids      joinrelids;
    Relids      join_plus_commute;
    List       *joininfos;
    ListCell   *l;
 
    /* Compute the relid set for the join we are considering */
    joinrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);
    Assert(ojrelid != 0);
    joinrelids = bms_add_member(joinrelids, ojrelid);
 
    remove_rel_from_query(root, rel, -1, sjinfo, joinrelids);
 
    /*
     * Remove any joinquals referencing the rel from the joininfo lists.
     *
     * In some cases, a joinqual has to be put back after deleting its
     * reference to the target rel.  This can occur for pseudoconstant and
     * outerjoin-delayed quals, which can get marked as requiring the rel in
     * order to force them to be evaluated at or above the join.  We can't
     * just discard them, though.  Only quals that logically belonged to the
     * outer join being discarded should be removed from the query.
     *
     * We might encounter a qual that is a clone of a deletable qual with some
     * outer-join relids added (see deconstruct_distribute_oj_quals).  To
     * ensure we get rid of such clones as well, add the relids of all OJs
     * commutable with this one to the set we test against for
     * pushed-down-ness.
     */
    join_plus_commute = bms_union(joinrelids,
                                  sjinfo->commute_above_r);
    join_plus_commute = bms_add_members(join_plus_commute,
                                        sjinfo->commute_below_l);
 
    /*
     * We must make a copy of the rel's old joininfo list before starting the
     * loop, because otherwise remove_join_clause_from_rels would destroy the
     * list while we're scanning it.
     */
    joininfos = list_copy(rel->joininfo);
    foreach(l, joininfos)
    {
        RestrictInfo *rinfo = (RestrictInfo *) lfirst(l);
 
        remove_join_clause_from_rels(root, rinfo, rinfo->required_relids);
 
        if (RINFO_IS_PUSHED_DOWN(rinfo, join_plus_commute))
        {
            /*
             * There might be references to relid or ojrelid in the
             * RestrictInfo's relid sets, as a consequence of PHVs having had
             * ph_eval_at sets that include those.  We already checked above
             * that any such PHV is safe (and updated its ph_eval_at), so we
             * can just drop those references.
             */
            remove_rel_from_restrictinfo(rinfo, relid, ojrelid);
 
            /*
             * Cross-check that the clause itself does not reference the
             * target rel or join.
             */
#ifdef USE_ASSERT_CHECKING
            {
                Relids      clause_varnos = pull_varnos(root,
                                                        (Node *) rinfo->clause);
 
                Assert(!bms_is_member(relid, clause_varnos));
                Assert(!bms_is_member(ojrelid, clause_varnos));
            }
#endif
            /* Now throw it back into the joininfo lists */
            distribute_restrictinfo_to_rels(root, rinfo);
        }
    }
 
    /*
     * There may be references to the rel in root->fkey_list, but if so,
     * match_foreign_keys_to_quals() will get rid of them.
     */
 
    /*
     * Now remove the rel from the baserel array to prevent it from being
     * referenced again.  (We can't do this earlier because
     * remove_join_clause_from_rels will touch it.)
     */
    root->simple_rel_array[relid] = NULL;
    root->simple_rte_array[relid] = NULL;
 
    /* And nuke the RelOptInfo, just in case there's another access path */
    pfree(rel);
 
    /*
     * Now repeat construction of attr_needed bits coming from all other
     * sources.
     */
    rebuild_placeholder_attr_needed(root);
    rebuild_joinclause_attr_needed(root);
    rebuild_eclass_attr_needed(root);
    rebuild_lateral_attr_needed(root);
}
 
/*
 * Remove any references to relid or ojrelid from the RestrictInfo.
 *
 * We only bother to clean out bits in clause_relids and required_relids,
 * not nullingrel bits in contained Vars and PHVs.  (This might have to be
 * improved sometime.)  However, if the RestrictInfo contains an OR clause
 * we have to also clean up the sub-clauses.
 */
static void
remove_rel_from_restrictinfo(RestrictInfo *rinfo, int relid, int ojrelid)
{
    /*
     * initsplan.c is fairly cavalier about allowing RestrictInfos to share
     * relid sets with other RestrictInfos, and SpecialJoinInfos too.  Make
     * sure this RestrictInfo has its own relid sets before we modify them.
     * (In present usage, clause_relids is probably not shared, but
     * required_relids could be; let's not assume anything.)
     */
    rinfo->clause_relids = bms_copy(rinfo->clause_relids);
    rinfo->clause_relids = bms_del_member(rinfo->clause_relids, relid);
    rinfo->clause_relids = bms_del_member(rinfo->clause_relids, ojrelid);
    /* Likewise for required_relids */
    rinfo->required_relids = bms_copy(rinfo->required_relids);
    rinfo->required_relids = bms_del_member(rinfo->required_relids, relid);
    rinfo->required_relids = bms_del_member(rinfo->required_relids, ojrelid);
 
    /* If it's an OR, recurse to clean up sub-clauses */
    if (restriction_is_or_clause(rinfo))
    {
        ListCell   *lc;
 
        Assert(is_orclause(rinfo->orclause));
        foreach(lc, ((BoolExpr *) rinfo->orclause)->args)
        {
            Node       *orarg = (Node *) lfirst(lc);
 
            /* OR arguments should be ANDs or sub-RestrictInfos */
            if (is_andclause(orarg))
            {
                List       *andargs = ((BoolExpr *) orarg)->args;
                ListCell   *lc2;
 
                foreach(lc2, andargs)
                {
                    RestrictInfo *rinfo2 = lfirst_node(RestrictInfo, lc2);
 
                    remove_rel_from_restrictinfo(rinfo2, relid, ojrelid);
                }
            }
            else
            {
                RestrictInfo *rinfo2 = castNode(RestrictInfo, orarg);
 
                remove_rel_from_restrictinfo(rinfo2, relid, ojrelid);
            }
        }
    }
}
 
/*
 * Remove any references to relid or sjinfo->ojrelid (if sjinfo != NULL)
 * from the EquivalenceClass.
 *
 * Like remove_rel_from_restrictinfo, we don't worry about cleaning out
 * any nullingrel bits in contained Vars and PHVs.  (This might have to be
 * improved sometime.)  We do need to fix the EC and EM relid sets to ensure
 * that implied join equalities will be generated at the appropriate join
 * level(s).
 */
static void
remove_rel_from_eclass(EquivalenceClass *ec, SpecialJoinInfo *sjinfo,
                       int relid, int subst)
{
    ListCell   *lc;
 
    /* Fix up the EC's overall relids */
    ec->ec_relids = adjust_relid_set(ec->ec_relids, relid, subst);
    if (sjinfo != NULL)
        ec->ec_relids = adjust_relid_set(ec->ec_relids,
                                         sjinfo->ojrelid, subst);
 
    /*
     * We don't expect any EC child members to exist at this point.  Ensure
     * that's the case, otherwise, we might be getting asked to do something
     * this function hasn't been coded for.
     */
    Assert(ec->ec_childmembers == NULL);
 
    /*
     * Fix up the member expressions.  Any non-const member that ends with
     * empty em_relids must be a Var or PHV of the removed relation.  We don't
     * need it anymore, so we can drop it.
     */
    foreach(lc, ec->ec_members)
    {
        EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc);
 
        if (bms_is_member(relid, cur_em->em_relids) ||
            (sjinfo != NULL && bms_is_member(sjinfo->ojrelid,
                                             cur_em->em_relids)))
        {
            Assert(!cur_em->em_is_const);
            cur_em->em_relids = adjust_relid_set(cur_em->em_relids, relid, subst);
            if (sjinfo != NULL)
                cur_em->em_relids = adjust_relid_set(cur_em->em_relids,
                                                     sjinfo->ojrelid, subst);
            if (bms_is_empty(cur_em->em_relids))
                ec->ec_members = foreach_delete_current(ec->ec_members, lc);
        }
    }
 
    /* Fix up the source clauses, in case we can re-use them later */
    foreach(lc, ec->ec_sources)
    {
        RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
 
        if (sjinfo == NULL)
            ChangeVarNodesExtended((Node *) rinfo, relid, subst, 0,
                                   replace_relid_callback);
        else
            remove_rel_from_restrictinfo(rinfo, relid, sjinfo->ojrelid);
    }
 
    /*
     * Rather than expend code on fixing up any already-derived clauses, just
     * drop them.  (At this point, any such clauses would be base restriction
     * clauses, which we'd not need anymore anyway.)
     */
    ec_clear_derived_clauses(ec);
}
 
/*
 * Remove any occurrences of the target relid from a joinlist structure.
 *
 * It's easiest to build a whole new list structure, so we handle it that
 * way.  Efficiency is not a big deal here.
 *
 * *nremoved is incremented by the number of occurrences removed (there
 * should be exactly one, but the caller checks that).
 */
static List *
remove_rel_from_joinlist(List *joinlist, int relid, int *nremoved)
{
    List       *result = NIL;
    ListCell   *jl;
 
    foreach(jl, joinlist)
    {
        Node       *jlnode = (Node *) lfirst(jl);
 
        if (IsA(jlnode, RangeTblRef))
        {
            int         varno = ((RangeTblRef *) jlnode)->rtindex;
 
            if (varno == relid)
                (*nremoved)++;
            else
                result = lappend(result, jlnode);
        }
        else if (IsA(jlnode, List))
        {
            /* Recurse to handle subproblem */
            List       *sublist;
 
            sublist = remove_rel_from_joinlist((List *) jlnode,
                                               relid, nremoved);
            /* Avoid including empty sub-lists in the result */
            if (sublist)
                result = lappend(result, sublist);
        }
        else
        {
            elog(ERROR, "unrecognized joinlist node type: %d",
                 (int) nodeTag(jlnode));
        }
    }
 
    return result;
}
 
 
/*
 * reduce_unique_semijoins
 *      Check for semijoins that can be simplified to plain inner joins
 *      because the inner relation is provably unique for the join clauses.
 *
 * Ideally this would happen during reduce_outer_joins, but we don't have
 * enough information at that point.
 *
 * To perform the strength reduction when applicable, we need only delete
 * the semijoin's SpecialJoinInfo from root->join_info_list.  (We don't
 * bother fixing the join type attributed to it in the query jointree,
 * since that won't be consulted again.)
 */
void
reduce_unique_semijoins(PlannerInfo *root)
{
    ListCell   *lc;
 
    /*
     * Scan the join_info_list to find semijoins.
     */
    foreach(lc, root->join_info_list)
    {
        SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(lc);
        int         innerrelid;
        RelOptInfo *innerrel;
        Relids      joinrelids;
        List       *restrictlist;
 
        /*
         * Must be a semijoin to a single baserel, else we aren't going to be
         * able to do anything with it.
         */
        if (sjinfo->jointype != JOIN_SEMI)
            continue;
 
        if (!bms_get_singleton_member(sjinfo->min_righthand, &innerrelid))
            continue;
 
        innerrel = find_base_rel(root, innerrelid);
 
        /*
         * Before we trouble to run generate_join_implied_equalities, make a
         * quick check to eliminate cases in which we will surely be unable to
         * prove uniqueness of the innerrel.
         */
        if (!rel_supports_distinctness(root, innerrel))
            continue;
 
        /* Compute the relid set for the join we are considering */
        joinrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);
        Assert(sjinfo->ojrelid == 0);   /* SEMI joins don't have RT indexes */
 
        /*
         * Since we're only considering a single-rel RHS, any join clauses it
         * has must be clauses linking it to the semijoin's min_lefthand.  We
         * can also consider EC-derived join clauses.
         */
        restrictlist =
            list_concat(generate_join_implied_equalities(root,
                                                         joinrelids,
                                                         sjinfo->min_lefthand,
                                                         innerrel,
                                                         NULL),
                        innerrel->joininfo);
 
        /* Test whether the innerrel is unique for those clauses. */
        if (!innerrel_is_unique(root,
                                joinrelids, sjinfo->min_lefthand, innerrel,
                                JOIN_SEMI, restrictlist, true))
            continue;
 
        /* OK, remove the SpecialJoinInfo from the list. */
        root->join_info_list = foreach_delete_current(root->join_info_list, lc);
    }
}
 
 
/*
 * rel_supports_distinctness
 *      Could the relation possibly be proven distinct on some set of columns?
 *
 * This is effectively a pre-checking function for rel_is_distinct_for().
 * It must return true if rel_is_distinct_for() could possibly return true
 * with this rel, but it should not expend a lot of cycles.  The idea is
 * that callers can avoid doing possibly-expensive processing to compute
 * rel_is_distinct_for()'s argument lists if the call could not possibly
 * succeed.
 */
static bool
rel_supports_distinctness(PlannerInfo *root, RelOptInfo *rel)
{
    /* We only know about baserels ... */
    if (rel->reloptkind != RELOPT_BASEREL)
        return false;
    if (rel->rtekind == RTE_RELATION)
    {
        /*
         * For a plain relation, we only know how to prove uniqueness by
         * reference to unique indexes.  Make sure there's at least one
         * suitable unique index.  It must be immediately enforced, and not a
         * partial index. (Keep these conditions in sync with
         * relation_has_unique_index_for!)
         */
        ListCell   *lc;
 
        foreach(lc, rel->indexlist)
        {
            IndexOptInfo *ind = (IndexOptInfo *) lfirst(lc);
 
            if (ind->unique && ind->immediate && ind->indpred == NIL)
                return true;
        }
    }
    else if (rel->rtekind == RTE_SUBQUERY)
    {
        Query      *subquery = root->simple_rte_array[rel->relid]->subquery;
 
        /* Check if the subquery has any qualities that support distinctness */
        if (query_supports_distinctness(subquery))
            return true;
    }
    /* We have no proof rules for any other rtekinds. */
    return false;
}
 
/*
 * rel_is_distinct_for
 *      Does the relation return only distinct rows according to clause_list?
 *
 * clause_list is a list of join restriction clauses involving this rel and
 * some other one.  Return true if no two rows emitted by this rel could
 * possibly join to the same row of the other rel.
 *
 * The caller must have already determined that each condition is a
 * mergejoinable equality with an expression in this relation on one side, and
 * an expression not involving this relation on the other.  The transient
 * outer_is_left flag is used to identify which side references this relation:
 * left side if outer_is_left is false, right side if it is true.
 *
 * Note that the passed-in clause_list may be destructively modified!  This
 * is OK for current uses, because the clause_list is built by the caller for
 * the sole purpose of passing to this function.
 *
 * (*extra_clauses) to be set to the right sides of baserestrictinfo clauses,
 * looking like "x = const" if distinctness is derived from such clauses, not
 * joininfo clauses.  Pass NULL to the extra_clauses if this value is not
 * needed.
 */
static bool
rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel, List *clause_list,
                    List **extra_clauses)
{
    /*
     * We could skip a couple of tests here if we assume all callers checked
     * rel_supports_distinctness first, but it doesn't seem worth taking any
     * risk for.
     */
    if (rel->reloptkind != RELOPT_BASEREL)
        return false;
    if (rel->rtekind == RTE_RELATION)
    {
        /*
         * Examine the indexes to see if we have a matching unique index.
         * relation_has_unique_index_for automatically adds any usable
         * restriction clauses for the rel, so we needn't do that here.
         */
        if (relation_has_unique_index_for(root, rel, clause_list, extra_clauses))
            return true;
    }
    else if (rel->rtekind == RTE_SUBQUERY)
    {
        Index       relid = rel->relid;
        Query      *subquery = root->simple_rte_array[relid]->subquery;
        List       *colnos = NIL;
        List       *opids = NIL;
        ListCell   *l;
 
        /*
         * Build the argument lists for query_is_distinct_for: a list of
         * output column numbers that the query needs to be distinct over, and
         * a list of equality operators that the output columns need to be
         * distinct according to.
         *
         * (XXX we are not considering restriction clauses attached to the
         * subquery; is that worth doing?)
         */
        foreach(l, clause_list)
        {
            RestrictInfo *rinfo = lfirst_node(RestrictInfo, l);
            Oid         op;
            Var        *var;
 
            /*
             * Get the equality operator we need uniqueness according to.
             * (This might be a cross-type operator and thus not exactly the
             * same operator the subquery would consider; that's all right
             * since query_is_distinct_for can resolve such cases.)  The
             * caller's mergejoinability test should have selected only
             * OpExprs.
             */
            op = castNode(OpExpr, rinfo->clause)->opno;
 
            /* caller identified the inner side for us */
            if (rinfo->outer_is_left)
                var = (Var *) get_rightop(rinfo->clause);
            else
                var = (Var *) get_leftop(rinfo->clause);
 
            /*
             * We may ignore any RelabelType node above the operand.  (There
             * won't be more than one, since eval_const_expressions() has been
             * applied already.)
             */
            if (var && IsA(var, RelabelType))
                var = (Var *) ((RelabelType *) var)->arg;
 
            /*
             * If inner side isn't a Var referencing a subquery output column,
             * this clause doesn't help us.
             */
            if (!var || !IsA(var, Var) ||
                var->varno != relid || var->varlevelsup != 0)
                continue;
 
            colnos = lappend_int(colnos, var->varattno);
            opids = lappend_oid(opids, op);
        }
 
        if (query_is_distinct_for(subquery, colnos, opids))
            return true;
    }
    return false;
}
 
 
/*
 * query_supports_distinctness - could the query possibly be proven distinct
 *      on some set of output columns?
 *
 * This is effectively a pre-checking function for query_is_distinct_for().
 * It must return true if query_is_distinct_for() could possibly return true
 * with this query, but it should not expend a lot of cycles.  The idea is
 * that callers can avoid doing possibly-expensive processing to compute
 * query_is_distinct_for()'s argument lists if the call could not possibly
 * succeed.
 */
bool
query_supports_distinctness(Query *query)
{
    /* SRFs break distinctness except with DISTINCT, see below */
    if (query->hasTargetSRFs && query->distinctClause == NIL)
        return false;
 
    /* check for features we can prove distinctness with */
    if (query->distinctClause != NIL ||
        query->groupClause != NIL ||
        query->groupingSets != NIL ||
        query->hasAggs ||
        query->havingQual ||
        query->setOperations)
        return true;
 
    return false;
}
 
/*
 * query_is_distinct_for - does query never return duplicates of the
 *      specified columns?
 *
 * query is a not-yet-planned subquery (in current usage, it's always from
 * a subquery RTE, which the planner avoids scribbling on).
 *
 * colnos is an integer list of output column numbers (resno's).  We are
 * interested in whether rows consisting of just these columns are certain
 * to be distinct.  "Distinctness" is defined according to whether the
 * corresponding upper-level equality operators listed in opids would think
 * the values are distinct.  (Note: the opids entries could be cross-type
 * operators, and thus not exactly the equality operators that the subquery
 * would use itself.  We use equality_ops_are_compatible() to check
 * compatibility.  That looks at opfamily membership for index AMs that have
 * declared that they support consistent equality semantics within an
 * opfamily, and so should give trustworthy answers for all operators that we
 * might need to deal with here.)
 */
bool
query_is_distinct_for(Query *query, List *colnos, List *opids)
{
    ListCell   *l;
    Oid         opid;
 
    Assert(list_length(colnos) == list_length(opids));
 
    /*
     * DISTINCT (including DISTINCT ON) guarantees uniqueness if all the
     * columns in the DISTINCT clause appear in colnos and operator semantics
     * match.  This is true even if there are SRFs in the DISTINCT columns or
     * elsewhere in the tlist.
     */
    if (query->distinctClause)
    {
        foreach(l, query->distinctClause)
        {
            SortGroupClause *sgc = (SortGroupClause *) lfirst(l);
            TargetEntry *tle = get_sortgroupclause_tle(sgc,
                                                       query->targetList);
 
            opid = distinct_col_search(tle->resno, colnos, opids);
            if (!OidIsValid(opid) ||
                !equality_ops_are_compatible(opid, sgc->eqop))
                break;          /* exit early if no match */
        }
        if (l == NULL)          /* had matches for all? */
            return true;
    }
 
    /*
     * Otherwise, a set-returning function in the query's targetlist can
     * result in returning duplicate rows, despite any grouping that might
     * occur before tlist evaluation.  (If all tlist SRFs are within GROUP BY
     * columns, it would be safe because they'd be expanded before grouping.
     * But it doesn't currently seem worth the effort to check for that.)
     */
    if (query->hasTargetSRFs)
        return false;
 
    /*
     * Similarly, GROUP BY without GROUPING SETS guarantees uniqueness if all
     * the grouped columns appear in colnos and operator semantics match.
     */
    if (query->groupClause && !query->groupingSets)
    {
        foreach(l, query->groupClause)
        {
            SortGroupClause *sgc = (SortGroupClause *) lfirst(l);
            TargetEntry *tle = get_sortgroupclause_tle(sgc,
                                                       query->targetList);
 
            opid = distinct_col_search(tle->resno, colnos, opids);
            if (!OidIsValid(opid) ||
                !equality_ops_are_compatible(opid, sgc->eqop))
                break;          /* exit early if no match */
        }
        if (l == NULL)          /* had matches for all? */
            return true;
    }
    else if (query->groupingSets)
    {
        /*
         * If we have grouping sets with expressions, we probably don't have
         * uniqueness and analysis would be hard. Punt.
         */
        if (query->groupClause)
            return false;
 
        /*
         * If we have no groupClause (therefore no grouping expressions), we
         * might have one or many empty grouping sets. If there's just one,
         * then we're returning only one row and are certainly unique. But
         * otherwise, we know we're certainly not unique.
         */
        if (list_length(query->groupingSets) == 1 &&
            ((GroupingSet *) linitial(query->groupingSets))->kind == GROUPING_SET_EMPTY)
            return true;
        else
            return false;
    }
    else
    {
        /*
         * If we have no GROUP BY, but do have aggregates or HAVING, then the
         * result is at most one row so it's surely unique, for any operators.
         */
        if (query->hasAggs || query->havingQual)
            return true;
    }
 
    /*
     * UNION, INTERSECT, EXCEPT guarantee uniqueness of the whole output row,
     * except with ALL.
     */
    if (query->setOperations)
    {
        SetOperationStmt *topop = castNode(SetOperationStmt, query->setOperations);
 
        Assert(topop->op != SETOP_NONE);
 
        if (!topop->all)
        {
            ListCell   *lg;
 
            /* We're good if all the nonjunk output columns are in colnos */
            lg = list_head(topop->groupClauses);
            foreach(l, query->targetList)
            {
                TargetEntry *tle = (TargetEntry *) lfirst(l);
                SortGroupClause *sgc;
 
                if (tle->resjunk)
                    continue;   /* ignore resjunk columns */
 
                /* non-resjunk columns should have grouping clauses */
                Assert(lg != NULL);
                sgc = (SortGroupClause *) lfirst(lg);
                lg = lnext(topop->groupClauses, lg);
 
                opid = distinct_col_search(tle->resno, colnos, opids);
                if (!OidIsValid(opid) ||
                    !equality_ops_are_compatible(opid, sgc->eqop))
                    break;      /* exit early if no match */
            }
            if (l == NULL)      /* had matches for all? */
                return true;
        }
    }
 
    /*
     * XXX Are there any other cases in which we can easily see the result
     * must be distinct?
     *
     * If you do add more smarts to this function, be sure to update
     * query_supports_distinctness() to match.
     */
 
    return false;
}
 
/*
 * distinct_col_search - subroutine for query_is_distinct_for
 *
 * If colno is in colnos, return the corresponding element of opids,
 * else return InvalidOid.  (Ordinarily colnos would not contain duplicates,
 * but if it does, we arbitrarily select the first match.)
 */
static Oid
distinct_col_search(int colno, List *colnos, List *opids)
{
    ListCell   *lc1,
               *lc2;
 
    forboth(lc1, colnos, lc2, opids)
    {
        if (colno == lfirst_int(lc1))
            return lfirst_oid(lc2);
    }
    return InvalidOid;
}
 
 
/*
 * innerrel_is_unique
 *    Check if the innerrel provably contains at most one tuple matching any
 *    tuple from the outerrel, based on join clauses in the 'restrictlist'.
 *
 * We need an actual RelOptInfo for the innerrel, but it's sufficient to
 * identify the outerrel by its Relids.  This asymmetry supports use of this
 * function before joinrels have been built.  (The caller is expected to
 * also supply the joinrelids, just to save recalculating that.)
 *
 * The proof must be made based only on clauses that will be "joinquals"
 * rather than "otherquals" at execution.  For an inner join there's no
 * difference; but if the join is outer, we must ignore pushed-down quals,
 * as those will become "otherquals".  Note that this means the answer might
 * vary depending on whether IS_OUTER_JOIN(jointype); since we cache the
 * answer without regard to that, callers must take care not to call this
 * with jointypes that would be classified differently by IS_OUTER_JOIN().
 *
 * The actual proof is undertaken by is_innerrel_unique_for(); this function
 * is a frontend that is mainly concerned with caching the answers.
 * In particular, the force_cache argument allows overriding the internal
 * heuristic about whether to cache negative answers; it should be "true"
 * if making an inquiry that is not part of the normal bottom-up join search
 * sequence.
 */
bool
innerrel_is_unique(PlannerInfo *root,
                   Relids joinrelids,
                   Relids outerrelids,
                   RelOptInfo *innerrel,
                   JoinType jointype,
                   List *restrictlist,
                   bool force_cache)
{
    return innerrel_is_unique_ext(root, joinrelids, outerrelids, innerrel,
                                  jointype, restrictlist, force_cache, NULL);
}
 
/*
 * innerrel_is_unique_ext
 *    Do the same as innerrel_is_unique(), but also set to (*extra_clauses)
 *    additional clauses from a baserestrictinfo list used to prove the
 *    uniqueness.
 *
 * A non-NULL extra_clauses indicates that we're checking for self-join and
 * correspondingly dealing with filtered clauses.
 */
bool
innerrel_is_unique_ext(PlannerInfo *root,
                       Relids joinrelids,
                       Relids outerrelids,
                       RelOptInfo *innerrel,
                       JoinType jointype,
                       List *restrictlist,
                       bool force_cache,
                       List **extra_clauses)
{
    MemoryContext old_context;
    ListCell   *lc;
    UniqueRelInfo *uniqueRelInfo;
    List       *outer_exprs = NIL;
    bool        self_join = (extra_clauses != NULL);
 
    /* Certainly can't prove uniqueness when there are no joinclauses */
    if (restrictlist == NIL)
        return false;
 
    /*
     * Make a quick check to eliminate cases in which we will surely be unable
     * to prove uniqueness of the innerrel.
     */
    if (!rel_supports_distinctness(root, innerrel))
        return false;
 
    /*
     * Query the cache to see if we've managed to prove that innerrel is
     * unique for any subset of this outerrel.  For non-self-join search, we
     * don't need an exact match, as extra outerrels can't make the innerrel
     * any less unique (or more formally, the restrictlist for a join to a
     * superset outerrel must be a superset of the conditions we successfully
     * used before). For self-join search, we require an exact match of
     * outerrels because we need extra clauses to be valid for our case. Also,
     * for self-join checking we've filtered the clauses list.  Thus, we can
     * match only the result cached for a self-join search for another
     * self-join check.
     */
    foreach(lc, innerrel->unique_for_rels)
    {
        uniqueRelInfo = (UniqueRelInfo *) lfirst(lc);
 
        if ((!self_join && bms_is_subset(uniqueRelInfo->outerrelids, outerrelids)) ||
            (self_join && bms_equal(uniqueRelInfo->outerrelids, outerrelids) &&
             uniqueRelInfo->self_join))
        {
            if (extra_clauses)
                *extra_clauses = uniqueRelInfo->extra_clauses;
            return true;        /* Success! */
        }
    }
 
    /*
     * Conversely, we may have already determined that this outerrel, or some
     * superset thereof, cannot prove this innerrel to be unique.
     */
    foreach(lc, innerrel->non_unique_for_rels)
    {
        Relids      unique_for_rels = (Relids) lfirst(lc);
 
        if (bms_is_subset(outerrelids, unique_for_rels))
            return false;
    }
 
    /* No cached information, so try to make the proof. */
    if (is_innerrel_unique_for(root, joinrelids, outerrelids, innerrel,
                               jointype, restrictlist,
                               self_join ? &outer_exprs : NULL))
    {
        /*
         * Cache the positive result for future probes, being sure to keep it
         * in the planner_cxt even if we are working in GEQO.
         *
         * Note: one might consider trying to isolate the minimal subset of
         * the outerrels that proved the innerrel unique.  But it's not worth
         * the trouble, because the planner builds up joinrels incrementally
         * and so we'll see the minimally sufficient outerrels before any
         * supersets of them anyway.
         */
        old_context = MemoryContextSwitchTo(root->planner_cxt);
        uniqueRelInfo = makeNode(UniqueRelInfo);
        uniqueRelInfo->outerrelids = bms_copy(outerrelids);
        uniqueRelInfo->self_join = self_join;
        uniqueRelInfo->extra_clauses = outer_exprs;
        innerrel->unique_for_rels = lappend(innerrel->unique_for_rels,
                                            uniqueRelInfo);
        MemoryContextSwitchTo(old_context);
 
        if (extra_clauses)
            *extra_clauses = outer_exprs;
        return true;            /* Success! */
    }
    else
    {
        /*
         * None of the join conditions for outerrel proved innerrel unique, so
         * we can safely reject this outerrel or any subset of it in future
         * checks.
         *
         * However, in normal planning mode, caching this knowledge is totally
         * pointless; it won't be queried again, because we build up joinrels
         * from smaller to larger.  It is useful in GEQO mode, where the
         * knowledge can be carried across successive planning attempts; and
         * it's likely to be useful when using join-search plugins, too. Hence
         * cache when join_search_private is non-NULL.  (Yeah, that's a hack,
         * but it seems reasonable.)
         *
         * Also, allow callers to override that heuristic and force caching;
         * that's useful for reduce_unique_semijoins, which calls here before
         * the normal join search starts.
         */
        if (force_cache || root->join_search_private)
        {
            old_context = MemoryContextSwitchTo(root->planner_cxt);
            innerrel->non_unique_for_rels =
                lappend(innerrel->non_unique_for_rels,
                        bms_copy(outerrelids));
            MemoryContextSwitchTo(old_context);
        }
 
        return false;
    }
}
 
/*
 * is_innerrel_unique_for
 *    Check if the innerrel provably contains at most one tuple matching any
 *    tuple from the outerrel, based on join clauses in the 'restrictlist'.
 */
static bool
is_innerrel_unique_for(PlannerInfo *root,
                       Relids joinrelids,
                       Relids outerrelids,
                       RelOptInfo *innerrel,
                       JoinType jointype,
                       List *restrictlist,
                       List **extra_clauses)
{
    List       *clause_list = NIL;
    ListCell   *lc;
 
    /*
     * Search for mergejoinable clauses that constrain the inner rel against
     * the outer rel.  If an operator is mergejoinable then it behaves like
     * equality for some btree opclass, so it's what we want.  The
     * mergejoinability test also eliminates clauses containing volatile
     * functions, which we couldn't depend on.
     */
    foreach(lc, restrictlist)
    {
        RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(lc);
 
        /*
         * As noted above, if it's a pushed-down clause and we're at an outer
         * join, we can't use it.
         */
        if (IS_OUTER_JOIN(jointype) &&
            RINFO_IS_PUSHED_DOWN(restrictinfo, joinrelids))
            continue;
 
        /* Ignore if it's not a mergejoinable clause */
        if (!restrictinfo->can_join ||
            restrictinfo->mergeopfamilies == NIL)
            continue;           /* not mergejoinable */
 
        /*
         * Check if the clause has the form "outer op inner" or "inner op
         * outer", and if so mark which side is inner.
         */
        if (!clause_sides_match_join(restrictinfo, outerrelids,
                                     innerrel->relids))
            continue;           /* no good for these input relations */
 
        /* OK, add to the list */
        clause_list = lappend(clause_list, restrictinfo);
    }
 
    /* Let rel_is_distinct_for() do the hard work */
    return rel_is_distinct_for(root, innerrel, clause_list, extra_clauses);
}
 
/*
 * Update EC members to point to the remaining relation instead of the removed
 * one, removing duplicates.
 *
 * Restriction clauses for base relations are already distributed to
 * the respective baserestrictinfo lists (see
 * generate_implied_equalities_for_column). The above code has already processed
 * this list and updated these clauses to reference the remaining
 * relation, so that we can skip them here based on their relids.
 *
 * Likewise, we have already processed the join clauses that join the
 * removed relation to the remaining one.
 *
 * Finally, there might be join clauses tying the removed relation to
 * some third relation.  We can't just delete the source clauses and
 * regenerate them from the EC because the corresponding equality
 * operators might be missing (see the handling of ec_broken).
 * Therefore, we will update the references in the source clauses.
 *
 * Derived clauses can be generated again, so it is simpler just to
 * delete them.
 */
static void
update_eclasses(EquivalenceClass *ec, int from, int to)
{
    List       *new_members = NIL;
    List       *new_sources = NIL;
 
    /*
     * We don't expect any EC child members to exist at this point.  Ensure
     * that's the case, otherwise, we might be getting asked to do something
     * this function hasn't been coded for.
     */
    Assert(ec->ec_childmembers == NULL);
 
    foreach_node(EquivalenceMember, em, ec->ec_members)
    {
        bool        is_redundant = false;
 
        if (!bms_is_member(from, em->em_relids))
        {
            new_members = lappend(new_members, em);
            continue;
        }
 
        em->em_relids = adjust_relid_set(em->em_relids, from, to);
        em->em_jdomain->jd_relids = adjust_relid_set(em->em_jdomain->jd_relids, from, to);
 
        /* We only process inner joins */
        ChangeVarNodesExtended((Node *) em->em_expr, from, to, 0,
                               replace_relid_callback);
 
        foreach_node(EquivalenceMember, other, new_members)
        {
            if (!equal(em->em_relids, other->em_relids))
                continue;
 
            if (equal(em->em_expr, other->em_expr))
            {
                is_redundant = true;
                break;
            }
        }
 
        if (!is_redundant)
            new_members = lappend(new_members, em);
    }
 
    list_free(ec->ec_members);
    ec->ec_members = new_members;
 
    ec_clear_derived_clauses(ec);
 
    /* Update EC source expressions */
    foreach_node(RestrictInfo, rinfo, ec->ec_sources)
    {
        bool        is_redundant = false;
 
        if (!bms_is_member(from, rinfo->required_relids))
        {
            new_sources = lappend(new_sources, rinfo);
            continue;
        }
 
        ChangeVarNodesExtended((Node *) rinfo, from, to, 0,
                               replace_relid_callback);
 
        /*
         * After switching the clause to the remaining relation, check it for
         * redundancy with existing ones. We don't have to check for
         * redundancy with derived clauses, because we've just deleted them.
         */
        foreach_node(RestrictInfo, other, new_sources)
        {
            if (!equal(rinfo->clause_relids, other->clause_relids))
                continue;
 
            if (equal(rinfo->clause, other->clause))
            {
                is_redundant = true;
                break;
            }
        }
 
        if (!is_redundant)
            new_sources = lappend(new_sources, rinfo);
    }
 
    list_free(ec->ec_sources);
    ec->ec_sources = new_sources;
    ec->ec_relids = adjust_relid_set(ec->ec_relids, from, to);
}
 
/*
 * "Logically" compares two RestrictInfo's ignoring the 'rinfo_serial' field,
 * which makes almost every RestrictInfo unique.  This type of comparison is
 * useful when removing duplicates while moving RestrictInfo's from removed
 * relation to remaining relation during self-join elimination.
 *
 * XXX: In the future, we might remove the 'rinfo_serial' field completely and
 * get rid of this function.
 */
static bool
restrict_infos_logically_equal(RestrictInfo *a, RestrictInfo *b)
{
    int         saved_rinfo_serial = a->rinfo_serial;
    bool        result;
 
    a->rinfo_serial = b->rinfo_serial;
    result = equal(a, b);
    a->rinfo_serial = saved_rinfo_serial;
 
    return result;
}
 
/*
 * This function adds all non-redundant clauses to the keeping relation
 * during self-join elimination.  That is a contradictory operation. On the
 * one hand, we reduce the length of the `restrict` lists, which can
 * impact planning or executing time.  Additionally, we improve the
 * accuracy of cardinality estimation.  On the other hand, it is one more
 * place that can make planning time much longer in specific cases.  It
 * would have been better to avoid calling the equal() function here, but
 * it's the only way to detect duplicated inequality expressions.
 *
 * (*keep_rinfo_list) is given by pointer because it might be altered by
 * distribute_restrictinfo_to_rels().
 */
static void
add_non_redundant_clauses(PlannerInfo *root,
                          List *rinfo_candidates,
                          List **keep_rinfo_list,
                          Index removed_relid)
{
    foreach_node(RestrictInfo, rinfo, rinfo_candidates)
    {
        bool        is_redundant = false;
 
        Assert(!bms_is_member(removed_relid, rinfo->required_relids));
 
        foreach_node(RestrictInfo, src, (*keep_rinfo_list))
        {
            if (!bms_equal(src->clause_relids, rinfo->clause_relids))
                /* Can't compare trivially different clauses */
                continue;
 
            if (src == rinfo ||
                (rinfo->parent_ec != NULL &&
                 src->parent_ec == rinfo->parent_ec) ||
                restrict_infos_logically_equal(rinfo, src))
            {
                is_redundant = true;
                break;
            }
        }
        if (!is_redundant)
            distribute_restrictinfo_to_rels(root, rinfo);
    }
}
 
/*
 * A custom callback for ChangeVarNodesExtended() providing
 * Self-join elimination (SJE) related functionality
 *
 * SJE needs to skip the RangeTblRef node
 * type.  During SJE's last step, remove_rel_from_joinlist() removes
 * remaining RangeTblRefs with target relid.  If ChangeVarNodes() replaces
 * the target relid before, remove_rel_from_joinlist() fails to identify
 * the nodes to delete.
 *
 * SJE also needs to change the relids within RestrictInfo's.
 */
static bool
replace_relid_callback(Node *node, ChangeVarNodes_context *context)
{
    if (IsA(node, RangeTblRef))
    {
        return true;
    }
    else if (IsA(node, RestrictInfo))
    {
        RestrictInfo *rinfo = (RestrictInfo *) node;
        int         relid = -1;
        bool        is_req_equal =
            (rinfo->required_relids == rinfo->clause_relids);
        bool        clause_relids_is_multiple =
            (bms_membership(rinfo->clause_relids) == BMS_MULTIPLE);
 
        /*
         * Recurse down into clauses if the target relation is present in
         * clause_relids or required_relids.  We must check required_relids
         * because the relation not present in clause_relids might still be
         * present somewhere in orclause.
         */
        if (bms_is_member(context->rt_index, rinfo->clause_relids) ||
            bms_is_member(context->rt_index, rinfo->required_relids))
        {
            Relids      new_clause_relids;
 
            ChangeVarNodesWalkExpression((Node *) rinfo->clause, context);
            ChangeVarNodesWalkExpression((Node *) rinfo->orclause, context);
 
            new_clause_relids = adjust_relid_set(rinfo->clause_relids,
                                                 context->rt_index,
                                                 context->new_index);
 
            /*
             * Incrementally adjust num_base_rels based on the change of
             * clause_relids, which could contain both base relids and
             * outer-join relids.  This operation is legal until we remove
             * only baserels.
             */
            rinfo->num_base_rels -= bms_num_members(rinfo->clause_relids) -
                bms_num_members(new_clause_relids);
 
            rinfo->clause_relids = new_clause_relids;
            rinfo->left_relids =
                adjust_relid_set(rinfo->left_relids, context->rt_index, context->new_index);
            rinfo->right_relids =
                adjust_relid_set(rinfo->right_relids, context->rt_index, context->new_index);
        }
 
        if (is_req_equal)
            rinfo->required_relids = rinfo->clause_relids;
        else
            rinfo->required_relids =
                adjust_relid_set(rinfo->required_relids, context->rt_index, context->new_index);
 
        rinfo->outer_relids =
            adjust_relid_set(rinfo->outer_relids, context->rt_index, context->new_index);
        rinfo->incompatible_relids =
            adjust_relid_set(rinfo->incompatible_relids, context->rt_index, context->new_index);
 
        if (rinfo->mergeopfamilies &&
            bms_get_singleton_member(rinfo->clause_relids, &relid) &&
            clause_relids_is_multiple &&
            relid == context->new_index && IsA(rinfo->clause, OpExpr))
        {
            Expr       *leftOp;
            Expr       *rightOp;
 
            leftOp = (Expr *) get_leftop(rinfo->clause);
            rightOp = (Expr *) get_rightop(rinfo->clause);
 
            /*
             * For self-join elimination, changing varnos could transform
             * "t1.a = t2.a" into "t1.a = t1.a".  That is always true as long
             * as "t1.a" is not null.  We use qual() to check for such a case,
             * and then we replace the qual for a check for not null
             * (NullTest).
             */
            if (leftOp != NULL && equal(leftOp, rightOp))
            {
                NullTest   *ntest = makeNode(NullTest);
 
                ntest->arg = leftOp;
                ntest->nulltesttype = IS_NOT_NULL;
                ntest->argisrow = false;
                ntest->location = -1;
                rinfo->clause = (Expr *) ntest;
                rinfo->mergeopfamilies = NIL;
                rinfo->left_em = NULL;
                rinfo->right_em = NULL;
            }
            Assert(rinfo->orclause == NULL);
        }
        return true;
    }
 
    return false;
}
 
/*
 * Remove a relation after we have proven that it participates only in an
 * unneeded unique self-join.
 *
 * Replace any links in planner info structures.
 *
 * Transfer join and restriction clauses from the removed relation to the
 * remaining one. We change the Vars of the clause to point to the
 * remaining relation instead of the removed one. The clauses that require
 * a subset of joinrelids become restriction clauses of the remaining
 * relation, and others remain join clauses. We append them to
 * baserestrictinfo and joininfo, respectively, trying not to introduce
 * duplicates.
 *
 * We also have to process the 'joinclauses' list here, because it
 * contains EC-derived join clauses which must become filter clauses. It
 * is not enough to just correct the ECs because the EC-derived
 * restrictions are generated before join removal (see
 * generate_base_implied_equalities).
 *
 * NOTE: Remember to keep the code in sync with PlannerInfo to be sure all
 * cached relids and relid bitmapsets can be correctly cleaned during the
 * self-join elimination procedure.
 */
static void
remove_self_join_rel(PlannerInfo *root, PlanRowMark *kmark, PlanRowMark *rmark,
                     RelOptInfo *toKeep, RelOptInfo *toRemove,
                     List *restrictlist)
{
    List       *joininfos;
    ListCell   *lc;
    int         i;
    List       *jinfo_candidates = NIL;
    List       *binfo_candidates = NIL;
 
    Assert(toKeep->relid > 0);
    Assert(toRemove->relid > 0);
 
    /*
     * Replace the index of the removing table with the keeping one. The
     * technique of removing/distributing restrictinfo is used here to attach
     * just appeared (for keeping relation) join clauses and avoid adding
     * duplicates of those that already exist in the joininfo list.
     */
    joininfos = list_copy(toRemove->joininfo);
    foreach_node(RestrictInfo, rinfo, joininfos)
    {
        remove_join_clause_from_rels(root, rinfo, rinfo->required_relids);
        ChangeVarNodesExtended((Node *) rinfo, toRemove->relid, toKeep->relid,
                               0, replace_relid_callback);
 
        if (bms_membership(rinfo->required_relids) == BMS_MULTIPLE)
            jinfo_candidates = lappend(jinfo_candidates, rinfo);
        else
            binfo_candidates = lappend(binfo_candidates, rinfo);
    }
 
    /*
     * Concatenate restrictlist to the list of base restrictions of the
     * removing table just to simplify the replacement procedure: all of them
     * weren't connected to any keeping relations and need to be added to some
     * rels.
     */
    toRemove->baserestrictinfo = list_concat(toRemove->baserestrictinfo,
                                             restrictlist);
    foreach_node(RestrictInfo, rinfo, toRemove->baserestrictinfo)
    {
        ChangeVarNodesExtended((Node *) rinfo, toRemove->relid, toKeep->relid,
                               0, replace_relid_callback);
 
        if (bms_membership(rinfo->required_relids) == BMS_MULTIPLE)
            jinfo_candidates = lappend(jinfo_candidates, rinfo);
        else
            binfo_candidates = lappend(binfo_candidates, rinfo);
    }
 
    /*
     * Now, add all non-redundant clauses to the keeping relation.
     */
    add_non_redundant_clauses(root, binfo_candidates,
                              &toKeep->baserestrictinfo, toRemove->relid);
    add_non_redundant_clauses(root, jinfo_candidates,
                              &toKeep->joininfo, toRemove->relid);
 
    list_free(binfo_candidates);
    list_free(jinfo_candidates);
 
    /*
     * Arrange equivalence classes, mentioned removing a table, with the
     * keeping one: varno of removing table should be replaced in members and
     * sources lists. Also, remove duplicated elements if this replacement
     * procedure created them.
     */
    i = -1;
    while ((i = bms_next_member(toRemove->eclass_indexes, i)) >= 0)
    {
        EquivalenceClass *ec = (EquivalenceClass *) list_nth(root->eq_classes, i);
 
        update_eclasses(ec, toRemove->relid, toKeep->relid);
        toKeep->eclass_indexes = bms_add_member(toKeep->eclass_indexes, i);
    }
 
    /*
     * Transfer the targetlist and attr_needed flags.
     */
 
    foreach(lc, toRemove->reltarget->exprs)
    {
        Node       *node = lfirst(lc);
 
        ChangeVarNodesExtended(node, toRemove->relid, toKeep->relid, 0,
                               replace_relid_callback);
        if (!list_member(toKeep->reltarget->exprs, node))
            toKeep->reltarget->exprs = lappend(toKeep->reltarget->exprs, node);
    }
 
    for (i = toKeep->min_attr; i <= toKeep->max_attr; i++)
    {
        int         attno = i - toKeep->min_attr;
 
        toRemove->attr_needed[attno] = adjust_relid_set(toRemove->attr_needed[attno],
                                                        toRemove->relid, toKeep->relid);
        toKeep->attr_needed[attno] = bms_add_members(toKeep->attr_needed[attno],
                                                     toRemove->attr_needed[attno]);
    }
 
    /*
     * If the removed relation has a row mark, transfer it to the remaining
     * one.
     *
     * If both rels have row marks, just keep the one corresponding to the
     * remaining relation because we verified earlier that they have the same
     * strength.
     */
    if (rmark)
    {
        if (kmark)
        {
            Assert(kmark->markType == rmark->markType);
 
            root->rowMarks = list_delete_ptr(root->rowMarks, rmark);
        }
        else
        {
            /* Shouldn't have inheritance children here. */
            Assert(rmark->rti == rmark->prti);
 
            rmark->rti = rmark->prti = toKeep->relid;
        }
    }
 
    /*
     * Replace varno in all the query structures, except nodes RangeTblRef
     * otherwise later remove_rel_from_joinlist will yield errors.
     */
    ChangeVarNodesExtended((Node *) root->parse, toRemove->relid, toKeep->relid,
                           0, replace_relid_callback);
 
    /* Replace links in the planner info */
    remove_rel_from_query(root, toRemove, toKeep->relid, NULL, NULL);
 
    /* At last, replace varno in root targetlist and HAVING clause */
    ChangeVarNodesExtended((Node *) root->processed_tlist, toRemove->relid,
                           toKeep->relid, 0, replace_relid_callback);
    ChangeVarNodesExtended((Node *) root->processed_groupClause,
                           toRemove->relid, toKeep->relid, 0,
                           replace_relid_callback);
 
    adjust_relid_set(root->all_result_relids, toRemove->relid, toKeep->relid);
    adjust_relid_set(root->leaf_result_relids, toRemove->relid, toKeep->relid);
 
    /*
     * There may be references to the rel in root->fkey_list, but if so,
     * match_foreign_keys_to_quals() will get rid of them.
     */
 
    /*
     * Finally, remove the rel from the baserel array to prevent it from being
     * referenced again.  (We can't do this earlier because
     * remove_join_clause_from_rels will touch it.)
     */
    root->simple_rel_array[toRemove->relid] = NULL;
    root->simple_rte_array[toRemove->relid] = NULL;
 
    /* And nuke the RelOptInfo, just in case there's another access path. */
    pfree(toRemove);
 
 
    /*
     * Now repeat construction of attr_needed bits coming from all other
     * sources.
     */
    rebuild_placeholder_attr_needed(root);
    rebuild_joinclause_attr_needed(root);
    rebuild_eclass_attr_needed(root);
    rebuild_lateral_attr_needed(root);
}
 
/*
 * split_selfjoin_quals
 *      Processes 'joinquals' by building two lists: one containing the quals
 *      where the columns/exprs are on either side of the join match and
 *      another one containing the remaining quals.
 *
 * 'joinquals' must only contain quals for a RTE_RELATION being joined to
 * itself.
 */
static void
split_selfjoin_quals(PlannerInfo *root, List *joinquals, List **selfjoinquals,
                     List **otherjoinquals, int from, int to)
{
    List       *sjoinquals = NIL;
    List       *ojoinquals = NIL;
 
    foreach_node(RestrictInfo, rinfo, joinquals)
    {
        OpExpr     *expr;
        Node       *leftexpr;
        Node       *rightexpr;
 
        /* In general, clause looks like F(arg1) = G(arg2) */
        if (!rinfo->mergeopfamilies ||
            bms_num_members(rinfo->clause_relids) != 2 ||
            bms_membership(rinfo->left_relids) != BMS_SINGLETON ||
            bms_membership(rinfo->right_relids) != BMS_SINGLETON)
        {
            ojoinquals = lappend(ojoinquals, rinfo);
            continue;
        }
 
        expr = (OpExpr *) rinfo->clause;
 
        if (!IsA(expr, OpExpr) || list_length(expr->args) != 2)
        {
            ojoinquals = lappend(ojoinquals, rinfo);
            continue;
        }
 
        leftexpr = get_leftop(rinfo->clause);
        rightexpr = copyObject(get_rightop(rinfo->clause));
 
        if (leftexpr && IsA(leftexpr, RelabelType))
            leftexpr = (Node *) ((RelabelType *) leftexpr)->arg;
        if (rightexpr && IsA(rightexpr, RelabelType))
            rightexpr = (Node *) ((RelabelType *) rightexpr)->arg;
 
        /*
         * Quite an expensive operation, narrowing the use case. For example,
         * when we have cast of the same var to different (but compatible)
         * types.
         */
        ChangeVarNodesExtended(rightexpr,
                               bms_singleton_member(rinfo->right_relids),
                               bms_singleton_member(rinfo->left_relids), 0,
                               replace_relid_callback);
 
        if (equal(leftexpr, rightexpr))
            sjoinquals = lappend(sjoinquals, rinfo);
        else
            ojoinquals = lappend(ojoinquals, rinfo);
    }
 
    *selfjoinquals = sjoinquals;
    *otherjoinquals = ojoinquals;
}
 
/*
 * Check for a case when uniqueness is at least partly derived from a
 * baserestrictinfo clause. In this case, we have a chance to return only
 * one row (if such clauses on both sides of SJ are equal) or nothing (if they
 * are different).
 */
static bool
match_unique_clauses(PlannerInfo *root, RelOptInfo *outer, List *uclauses,
                     Index relid)
{
    foreach_node(RestrictInfo, rinfo, uclauses)
    {
        Expr       *clause;
        Node       *iclause;
        Node       *c1;
        bool        matched = false;
 
        Assert(outer->relid > 0 && relid > 0);
 
        /* Only filters like f(R.x1,...,R.xN) == expr we should consider. */
        Assert(bms_is_empty(rinfo->left_relids) ^
               bms_is_empty(rinfo->right_relids));
 
        clause = (Expr *) copyObject(rinfo->clause);
        ChangeVarNodesExtended((Node *) clause, relid, outer->relid, 0,
                               replace_relid_callback);
 
        iclause = bms_is_empty(rinfo->left_relids) ? get_rightop(clause) :
            get_leftop(clause);
        c1 = bms_is_empty(rinfo->left_relids) ? get_leftop(clause) :
            get_rightop(clause);
 
        /*
         * Compare these left and right sides with the corresponding sides of
         * the outer's filters. If no one is detected - return immediately.
         */
        foreach_node(RestrictInfo, orinfo, outer->baserestrictinfo)
        {
            Node       *oclause;
            Node       *c2;
 
            if (orinfo->mergeopfamilies == NIL)
                /* Don't consider clauses that aren't similar to 'F(X)=G(Y)' */
                continue;
 
            Assert(is_opclause(orinfo->clause));
 
            oclause = bms_is_empty(orinfo->left_relids) ?
                get_rightop(orinfo->clause) : get_leftop(orinfo->clause);
            c2 = (bms_is_empty(orinfo->left_relids) ?
                  get_leftop(orinfo->clause) : get_rightop(orinfo->clause));
 
            if (equal(iclause, oclause) && equal(c1, c2))
            {
                matched = true;
                break;
            }
        }
 
        if (!matched)
            return false;
    }
 
    return true;
}
 
/*
 * Find and remove unique self-joins in a group of base relations that have
 * the same Oid.
 *
 * Returns a set of relids that were removed.
 */
static Relids
remove_self_joins_one_group(PlannerInfo *root, Relids relids)
{
    Relids      result = NULL;
    int         k;              /* Index of kept relation */
    int         r = -1;         /* Index of removed relation */
 
    while ((r = bms_next_member(relids, r)) > 0)
    {
        RelOptInfo *rrel = root->simple_rel_array[r];
 
        k = r;
 
        while ((k = bms_next_member(relids, k)) > 0)
        {
            Relids      joinrelids = NULL;
            RelOptInfo *krel = root->simple_rel_array[k];
            List       *restrictlist;
            List       *selfjoinquals;
            List       *otherjoinquals;
            ListCell   *lc;
            bool        jinfo_check = true;
            PlanRowMark *kmark = NULL;
            PlanRowMark *rmark = NULL;
            List       *uclauses = NIL;
 
            /* A sanity check: the relations have the same Oid. */
            Assert(root->simple_rte_array[k]->relid ==
                   root->simple_rte_array[r]->relid);
 
            /*
             * It is impossible to eliminate the join of two relations if they
             * belong to different rules of order. Otherwise, the planner
             * can't find any variants of the correct query plan.
             */
            foreach(lc, root->join_info_list)
            {
                SpecialJoinInfo *info = (SpecialJoinInfo *) lfirst(lc);
 
                if ((bms_is_member(k, info->syn_lefthand) ^
                     bms_is_member(r, info->syn_lefthand)) ||
                    (bms_is_member(k, info->syn_righthand) ^
                     bms_is_member(r, info->syn_righthand)))
                {
                    jinfo_check = false;
                    break;
                }
            }
            if (!jinfo_check)
                continue;
 
            /*
             * Check Row Marks equivalence. We can't remove the join if the
             * relations have row marks of different strength (e.g., one is
             * locked FOR UPDATE, and another just has ROW_MARK_REFERENCE for
             * EvalPlanQual rechecking).
             */
            foreach(lc, root->rowMarks)
            {
                PlanRowMark *rowMark = (PlanRowMark *) lfirst(lc);
 
                if (rowMark->rti == r)
                {
                    Assert(rmark == NULL);
                    rmark = rowMark;
                }
                else if (rowMark->rti == k)
                {
                    Assert(kmark == NULL);
                    kmark = rowMark;
                }
 
                if (kmark && rmark)
                    break;
            }
            if (kmark && rmark && kmark->markType != rmark->markType)
                continue;
 
            /*
             * We only deal with base rels here, so their relids bitset
             * contains only one member -- their relid.
             */
            joinrelids = bms_add_member(joinrelids, r);
            joinrelids = bms_add_member(joinrelids, k);
 
            /*
             * PHVs should not impose any constraints on removing self-joins.
             */
 
            /*
             * At this stage, joininfo lists of inner and outer can contain
             * only clauses required for a superior outer join that can't
             * influence this optimization. So, we can avoid to call the
             * build_joinrel_restrictlist() routine.
             */
            restrictlist = generate_join_implied_equalities(root, joinrelids,
                                                            rrel->relids,
                                                            krel, NULL);
            if (restrictlist == NIL)
                continue;
 
            /*
             * Process restrictlist to separate the self-join quals from the
             * other quals. e.g., "x = x" goes to selfjoinquals and "a = b" to
             * otherjoinquals.
             */
            split_selfjoin_quals(root, restrictlist, &selfjoinquals,
                                 &otherjoinquals, rrel->relid, krel->relid);
 
            Assert(list_length(restrictlist) ==
                   (list_length(selfjoinquals) + list_length(otherjoinquals)));
 
            /*
             * To enable SJE for the only degenerate case without any self
             * join clauses at all, add baserestrictinfo to this list. The
             * degenerate case works only if both sides have the same clause.
             * So doesn't matter which side to add.
             */
            selfjoinquals = list_concat(selfjoinquals, krel->baserestrictinfo);
 
            /*
             * Determine if the rrel can duplicate outer rows. We must bypass
             * the unique rel cache here since we're possibly using a subset
             * of join quals. We can use 'force_cache' == true when all join
             * quals are self-join quals.  Otherwise, we could end up putting
             * false negatives in the cache.
             */
            if (!innerrel_is_unique_ext(root, joinrelids, rrel->relids,
                                        krel, JOIN_INNER, selfjoinquals,
                                        list_length(otherjoinquals) == 0,
                                        &uclauses))
                continue;
 
            /*
             * 'uclauses' is the copy of outer->baserestrictinfo that are
             * associated with an index.  We proved by matching selfjoinquals
             * to a unique index that the outer relation has at most one
             * matching row for each inner row.  Sometimes that is not enough.
             * e.g. "WHERE s1.b = s2.b AND s1.a = 1 AND s2.a = 2" when the
             * unique index is (a,b).  Having non-empty uclauses, we must
             * validate that the inner baserestrictinfo contains the same
             * expressions, or we won't match the same row on each side of the
             * join.
             */
            if (!match_unique_clauses(root, rrel, uclauses, krel->relid))
                continue;
 
            /*
             * Remove rrel ReloptInfo from the planner structures and the
             * corresponding row mark.
             */
            remove_self_join_rel(root, kmark, rmark, krel, rrel, restrictlist);
 
            result = bms_add_member(result, r);
 
            /* We have removed the outer relation, try the next one. */
            break;
        }
    }
 
    return result;
}
 
/*
 * Gather indexes of base relations from the joinlist and try to eliminate self
 * joins.
 */
static Relids
remove_self_joins_recurse(PlannerInfo *root, List *joinlist, Relids toRemove)
{
    ListCell   *jl;
    Relids      relids = NULL;
    SelfJoinCandidate *candidates = NULL;
    int         i;
    int         j;
    int         numRels;
 
    /* Collect indexes of base relations of the join tree */
    foreach(jl, joinlist)
    {
        Node       *jlnode = (Node *) lfirst(jl);
 
        if (IsA(jlnode, RangeTblRef))
        {
            int         varno = ((RangeTblRef *) jlnode)->rtindex;
            RangeTblEntry *rte = root->simple_rte_array[varno];
 
            /*
             * We only consider ordinary relations as candidates to be
             * removed, and these relations should not have TABLESAMPLE
             * clauses specified.  Removing a relation with TABLESAMPLE clause
             * could potentially change the syntax of the query. Because of
             * UPDATE/DELETE EPQ mechanism, currently Query->resultRelation or
             * Query->mergeTargetRelation associated rel cannot be eliminated.
             */
            if (rte->rtekind == RTE_RELATION &&
                rte->relkind == RELKIND_RELATION &&
                rte->tablesample == NULL &&
                varno != root->parse->resultRelation &&
                varno != root->parse->mergeTargetRelation)
            {
                Assert(!bms_is_member(varno, relids));
                relids = bms_add_member(relids, varno);
            }
        }
        else if (IsA(jlnode, List))
        {
            /* Recursively go inside the sub-joinlist */
            toRemove = remove_self_joins_recurse(root, (List *) jlnode,
                                                 toRemove);
        }
        else
            elog(ERROR, "unrecognized joinlist node type: %d",
                 (int) nodeTag(jlnode));
    }
 
    numRels = bms_num_members(relids);
 
    /* Need at least two relations for the join */
    if (numRels < 2)
        return toRemove;
 
    /*
     * In order to find relations with the same oid we first build an array of
     * candidates and then sort it by oid.
     */
    candidates = (SelfJoinCandidate *) palloc(sizeof(SelfJoinCandidate) *
                                              numRels);
    i = -1;
    j = 0;
    while ((i = bms_next_member(relids, i)) >= 0)
    {
        candidates[j].relid = i;
        candidates[j].reloid = root->simple_rte_array[i]->relid;
        j++;
    }
 
    qsort(candidates, numRels, sizeof(SelfJoinCandidate),
          self_join_candidates_cmp);
 
    /*
     * Iteratively form a group of relation indexes with the same oid and
     * launch the routine that detects self-joins in this group and removes
     * excessive range table entries.
     *
     * At the end of the iteration, exclude the group from the overall relids
     * list. So each next iteration of the cycle will involve less and less
     * value of relids.
     */
    i = 0;
    for (j = 1; j < numRels + 1; j++)
    {
        if (j == numRels || candidates[j].reloid != candidates[i].reloid)
        {
            if (j - i >= 2)
            {
                /* Create a group of relation indexes with the same oid */
                Relids      group = NULL;
                Relids      removed;
 
                while (i < j)
                {
                    group = bms_add_member(group, candidates[i].relid);
                    i++;
                }
                relids = bms_del_members(relids, group);
 
                /*
                 * Try to remove self-joins from a group of identical entries.
                 * Make the next attempt iteratively - if something is deleted
                 * from a group, changes in clauses and equivalence classes
                 * can give us a chance to find more candidates.
                 */
                do
                {
                    Assert(!bms_overlap(group, toRemove));
                    removed = remove_self_joins_one_group(root, group);
                    toRemove = bms_add_members(toRemove, removed);
                    group = bms_del_members(group, removed);
                } while (!bms_is_empty(removed) &&
                         bms_membership(group) == BMS_MULTIPLE);
                bms_free(removed);
                bms_free(group);
            }
            else
            {
                /* Single relation, just remove it from the set */
                relids = bms_del_member(relids, candidates[i].relid);
                i = j;
            }
        }
    }
 
    Assert(bms_is_empty(relids));
 
    return toRemove;
}
 
/*
 * Compare self-join candidates by their oids.
 */
static int
self_join_candidates_cmp(const void *a, const void *b)
{
    const SelfJoinCandidate *ca = (const SelfJoinCandidate *) a;
    const SelfJoinCandidate *cb = (const SelfJoinCandidate *) b;
 
    if (ca->reloid != cb->reloid)
        return (ca->reloid < cb->reloid ? -1 : 1);
    else
        return 0;
}
 
/*
 * Find and remove useless self joins.
 *
 * Search for joins where a relation is joined to itself. If the join clause
 * for each tuple from one side of the join is proven to match the same
 * physical row (or nothing) on the other side, that self-join can be
 * eliminated from the query.  Suitable join clauses are assumed to be in the
 * form of X = X, and can be replaced with NOT NULL clauses.
 *
 * For the sake of simplicity, we don't apply this optimization to special
 * joins. Here is a list of what we could do in some particular cases:
 * 'a a1 semi join a a2': is reduced to inner by reduce_unique_semijoins,
 * and then removed normally.
 * 'a a1 anti join a a2': could simplify to a scan with 'outer quals AND
 * (IS NULL on join columns OR NOT inner quals)'.
 * 'a a1 left join a a2': could simplify to a scan like inner but without
 * NOT NULL conditions on join columns.
 * 'a a1 left join (a a2 join b)': can't simplify this, because join to b
 * can both remove rows and introduce duplicates.
 *
 * To search for removable joins, we order all the relations on their Oid,
 * go over each set with the same Oid, and consider each pair of relations
 * in this set.
 *
 * To remove the join, we mark one of the participating relations as dead
 * and rewrite all references to it to point to the remaining relation.
 * This includes modifying RestrictInfos, EquivalenceClasses, and
 * EquivalenceMembers. We also have to modify the row marks. The join clauses
 * of the removed relation become either restriction or join clauses, based on
 * whether they reference any relations not participating in the removed join.
 *
 * 'joinlist' is the top-level joinlist of the query. If it has any
 * references to the removed relations, we update them to point to the
 * remaining ones.
 */
List *
remove_useless_self_joins(PlannerInfo *root, List *joinlist)
{
    Relids      toRemove = NULL;
    int         relid = -1;
 
    if (!enable_self_join_elimination || joinlist == NIL ||
        (list_length(joinlist) == 1 && !IsA(linitial(joinlist), List)))
        return joinlist;
 
    /*
     * Merge pairs of relations participated in self-join. Remove unnecessary
     * range table entries.
     */
    toRemove = remove_self_joins_recurse(root, joinlist, toRemove);
 
    if (unlikely(toRemove != NULL))
    {
        /* At the end, remove orphaned relation links */
        while ((relid = bms_next_member(toRemove, relid)) >= 0)
        {
            int         nremoved = 0;
 
            joinlist = remove_rel_from_joinlist(joinlist, relid, &nremoved);
            if (nremoved != 1)
                elog(ERROR, "failed to find relation %d in joinlist", relid);
        }
    }
 
    return joinlist;
}