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Richard Guo
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Expand virtual generated columns for ALTER COLUMN TYPE
For the subcommand ALTER COLUMN TYPE of the ALTER TABLE command, the USING expression may reference virtual generated columns. These columns must be expanded before the expression is fed through expression_planner and the expression-execution machinery. Failing to do so can result in incorrect rewrite decisions, and can also lead to "ERROR: unexpected virtual generated column reference". Reported-by: Alexander Lakhin <[email protected]> Reviewed-by: jian he <[email protected]> Discussion: https://postgr.es/m/[email protected]
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-19
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3 files changed

+30
-19
lines changed

‎src/backend/commands/tablecmds.c

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -14484,6 +14484,9 @@ ATPrepAlterColumnType(List **wqueue,
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/* Fix collations after all else */
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assign_expr_collations(pstate, transform);
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/* Expand virtual generated columns in the expr. */
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transform = expand_generated_columns_in_expr(transform, rel, 1);
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/* Plan the expr now so we can accurately assess the need to rewrite. */
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transform = (Node *) expression_planner((Expr *) transform);
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‎src/test/regress/expected/generated_virtual.out

Lines changed: 20 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -1479,7 +1479,8 @@ create table gtest32 (
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a int primary key,
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b int generated always as (a * 2),
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c int generated always as (10 + 10),
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d int generated always as (coalesce(a, 100))
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d int generated always as (coalesce(a, 100)),
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e int
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);
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insert into gtest32 values (1), (2);
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analyze gtest32;
@@ -1563,41 +1564,44 @@ select t2.* from gtest32 t1 left join gtest32 t2 on false;
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QUERY PLAN
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------------------------------------------------------
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Nested Loop Left Join
1566-
Output: a, (a * 2), (20), (COALESCE(a, 100))
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Output: a, (a * 2), (20), (COALESCE(a, 100)), e
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Join Filter: false
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-> Seq Scan on generated_virtual_tests.gtest32 t1
1569-
Output: t1.a, t1.b, t1.c, t1.d
1570+
Output: t1.a, t1.b, t1.c, t1.d, t1.e
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-> Result
1571-
Output: a, 20, COALESCE(a, 100)
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Output: a, e, 20, COALESCE(a, 100)
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One-Time Filter: false
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(8 rows)
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select t2.* from gtest32 t1 left join gtest32 t2 on false;
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a | b | c | d
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---+---+---+---
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| | |
1579-
| | |
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a | b | c | d | e
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---+---+---+---+---
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| | | |
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| | | |
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(2 rows)
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explain (verbose, costs off)
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select * from gtest32 t group by grouping sets (a, b, c, d) having c = 20;
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select * from gtest32 t group by grouping sets (a, b, c, d, e) having c = 20;
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QUERY PLAN
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-----------------------------------------------------
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HashAggregate
1587-
Output: a, ((a * 2)), (20), (COALESCE(a, 100))
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Output: a, ((a * 2)), (20), (COALESCE(a, 100)), e
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Hash Key: t.a
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Hash Key: (t.a * 2)
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Hash Key: 20
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Hash Key: COALESCE(t.a, 100)
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Hash Key: t.e
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Filter: ((20) = 20)
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-> Seq Scan on generated_virtual_tests.gtest32 t
1594-
Output: a, (a * 2), 20, COALESCE(a, 100)
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(9 rows)
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Output: a, (a * 2), 20, COALESCE(a, 100), e
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(10 rows)
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1597-
select * from gtest32 t group by grouping sets (a, b, c, d) having c = 20;
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a | b | c | d
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---+---+----+---
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| | 20 |
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select * from gtest32 t group by grouping sets (a, b, c, d, e) having c = 20;
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a | b | c | d | e
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---+---+----+---+---
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| | 20 | |
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(1 row)
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-- Ensure that the virtual generated columns in ALTER COLUMN TYPE USING expression are expanded
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alter table gtest32 alter column e type bigint using b;
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drop table gtest32;

‎src/test/regress/sql/generated_virtual.sql

Lines changed: 7 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -797,7 +797,8 @@ create table gtest32 (
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a int primary key,
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b int generated always as (a * 2),
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c int generated always as (10 + 10),
800-
d int generated always as (coalesce(a, 100))
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d int generated always as (coalesce(a, 100)),
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e int
801802
);
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insert into gtest32 values (1), (2);
@@ -838,7 +839,10 @@ select t2.* from gtest32 t1 left join gtest32 t2 on false;
838839
select t2.* from gtest32 t1 left join gtest32 t2 on false;
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840841
explain (verbose, costs off)
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select * from gtest32 t group by grouping sets (a, b, c, d) having c = 20;
842-
select * from gtest32 t group by grouping sets (a, b, c, d) having c = 20;
842+
select * from gtest32 t group by grouping sets (a, b, c, d, e) having c = 20;
843+
select * from gtest32 t group by grouping sets (a, b, c, d, e) having c = 20;
844+
845+
-- Ensure that the virtual generated columns in ALTER COLUMN TYPE USING expression are expanded
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alter table gtest32 alter column e type bigint using b;
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844848
drop table gtest32;

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