Improve speed of hash index build.

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

1 files changed, 17 insertions, 2 deletions

diff --git a/src/backend/utils/sort/tuplesortvariants.c b/src/backend/utils/sort/tuplesortvariants.c
index 2933020dcc8..7ad4429ad3d 100644
--- 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),
@@ -1410,6 +1413,18 @@ comparetup_index_hash(const SortTuple *a, const SortTuple *b,
 		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
 	 * scans in physical order.