Improve speed of hash index build.

tglsfdc · tglsfdc · commit e09d7a1262c6 · 2022-07-28T14:34:32.000-04:00
In the initial data sort, if the bucket numbers are the same then next sort on the hash value. Because index pages are kept in hash value order, this gains a little speed by allowing the eventual tuple insertions to be done sequentially, avoiding repeated data movement within PageAddItem. This seems to be good for overall speedup of 5%-9%, depending on the incoming data. Simon Riggs, reviewed by Amit Kapila Discussion: https://postgr.es/m/CANbhV-FG-1ZNMBuwhUF7AxxJz3u5137dYL-o6hchK1V_dMw86g@mail.gmail.com
diff --git a/src/backend/access/hash/hashsort.c b/src/backend/access/hash/hashsort.c
@@ -42,9 +42,10 @@ struct HSpool
 	Relation	index;
 
 	/*
-	 * We sort the hash keys based on the buckets they belong to. Below masks
-	 * are used in _hash_hashkey2bucket to determine the bucket of given hash
-	 * key.
+	 * We sort the hash keys based on the buckets they belong to, then by the
+	 * hash values themselves, to optimize insertions onto hash pages.  The
+	 * masks below are used in _hash_hashkey2bucket to determine the bucket of
+	 * a given hash key.
 	 */
 	uint32		high_mask;
 	uint32		low_mask;
diff --git a/src/backend/utils/sort/tuplesortvariants.c b/src/backend/utils/sort/tuplesortvariants.c
@@ -1387,14 +1387,17 @@ comparetup_index_hash(const SortTuple *a, const SortTuple *b,
 {
 	Bucket		bucket1;
 	Bucket		bucket2;
+	uint32		hash1;
+	uint32		hash2;
 	IndexTuple	tuple1;
 	IndexTuple	tuple2;
 	TuplesortPublic *base = TuplesortstateGetPublic(state);
 	TuplesortIndexHashArg *arg = (TuplesortIndexHashArg *) base->arg;
 
 	/*
-	 * Fetch hash keys and mask off bits we don't want to sort by. We know
-	 * that the first column of the index tuple is the hash key.
+	 * Fetch hash keys and mask off bits we don't want to sort by, so that the
+	 * initial sort is just on the bucket number.  We know that the first
+	 * column of the index tuple is the hash key.
 	 */
 	Assert(!a->isnull1);
 	bucket1 = _hash_hashkey2bucket(DatumGetUInt32(a->datum1),
@@ -1409,6 +1412,18 @@ comparetup_index_hash(const SortTuple *a, const SortTuple *b,
 	else if (bucket1 < bucket2)
 		return -1;
 
+	/*
+	 * If bucket values are equal, sort by hash values.  This allows us to
+	 * insert directly onto bucket/overflow pages, where the index tuples are
+	 * stored in hash order to allow fast binary search within each page.
+	 */
+	hash1 = DatumGetUInt32(a->datum1);
+	hash2 = DatumGetUInt32(b->datum1);
+	if (hash1 > hash2)
+		return 1;
+	else if (hash1 < hash2)
+		return -1;
+
 	/*
 	 * If hash values are equal, we sort on ItemPointer.  This does not affect
 	 * validity of the finished index, but it may be useful to have index
