Speedup hash index builds by skipping needless binary searches

When building hash indexes using the spool method, tuples are added to the
index page in hashkey order.  Because of this, we can safely skip
performing the binary search on the existing tuples on the page to find
the location to insert the tuple based on its hashkey value.  For this
case, we can just always put the tuple at the end of the item array as the
tuples will always arrive in hashkey order.

Testing has shown that this can improve hash index build speeds by 5-15%
with a unique set of integer values.

Author: Simon Riggs
Reviewed-by: David Rowley
Tested-by: David Zhang, Tomas Vondra
Discussion: https://postgr.es/m/CANbhV-GBc5JoG0AneUGPZZW3o4OK5LjBGeKe_icpC3R1McrZWQ@mail.gmail.com

src/backend/access/hash/hash.c

@@ -231,7 +231,7 @@ hashbuildCallback(Relation index,
 		itup = index_form_tuple(RelationGetDescr(index),
 								index_values, index_isnull);
 		itup->t_tid = *tid;
-		_hash_doinsert(index, itup, buildstate->heapRel);
+		_hash_doinsert(index, itup, buildstate->heapRel, false);
 		pfree(itup);
 	}
 
@@ -265,7 +265,7 @@ hashinsert(Relation rel, Datum *values, bool *isnull,
 	itup = index_form_tuple(RelationGetDescr(rel), index_values, index_isnull);
 	itup->t_tid = *ht_ctid;
 
-	_hash_doinsert(rel, itup, heapRel);
+	_hash_doinsert(rel, itup, heapRel, false);
 
 	pfree(itup);
 

src/backend/access/hash/hashinsert.c

@@ -32,9 +32,12 @@ static void _hash_vacuum_one_page(Relation rel, Relation hrel,
  *
  *		This routine is called by the public interface routines, hashbuild
  *		and hashinsert.  By here, itup is completely filled in.
+ *
+ * 'sorted' must only be passed as 'true' when inserts are done in hashkey
+ * order.
  */
 void
-_hash_doinsert(Relation rel, IndexTuple itup, Relation heapRel)
+_hash_doinsert(Relation rel, IndexTuple itup, Relation heapRel, bool sorted)
 {
 	Buffer		buf = InvalidBuffer;
 	Buffer		bucket_buf;
@@ -198,7 +201,7 @@ restart_insert:
 	START_CRIT_SECTION();
 
 	/* found page with enough space, so add the item here */
-	itup_off = _hash_pgaddtup(rel, buf, itemsz, itup);
+	itup_off = _hash_pgaddtup(rel, buf, itemsz, itup, sorted);
 	MarkBufferDirty(buf);
 
 	/* metapage operations */
@@ -263,21 +266,43 @@ restart_insert:
  *
  * Returns the offset number at which the tuple was inserted.  This function
  * is responsible for preserving the condition that tuples in a hash index
- * page are sorted by hashkey value.
+ * page are sorted by hashkey value, however, if the caller is certain that
+ * the hashkey for the tuple being added is >= the hashkeys of all existing
+ * tuples on the page, then the 'appendtup' flag may be passed as true.  This
+ * saves from having to binary search for the correct location to insert the
+ * tuple.
  */
 OffsetNumber
-_hash_pgaddtup(Relation rel, Buffer buf, Size itemsize, IndexTuple itup)
+_hash_pgaddtup(Relation rel, Buffer buf, Size itemsize, IndexTuple itup,
+			   bool appendtup)
 {
 	OffsetNumber itup_off;
 	Page		page;
-	uint32		hashkey;
 
 	_hash_checkpage(rel, buf, LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
 	page = BufferGetPage(buf);
 
-	/* Find where to insert the tuple (preserving page's hashkey ordering) */
-	hashkey = _hash_get_indextuple_hashkey(itup);
-	itup_off = _hash_binsearch(page, hashkey);
+	/*
+	 * Find where to insert the tuple (preserving page's hashkey ordering). If
+	 * 'appendtup' is true then we just insert it at the end.
+	 */
+	if (appendtup)
+	{
+		itup_off = PageGetMaxOffsetNumber(page) + 1;
+
+		/* ensure this tuple's hashkey is >= the final existing tuple */
+		Assert(PageGetMaxOffsetNumber(page) == 0 ||
+			   _hash_get_indextuple_hashkey((IndexTuple)
+											PageGetItem(page, PageGetItemId(page,
+																			PageGetMaxOffsetNumber(page)))) <=
+			   _hash_get_indextuple_hashkey(itup));
+	}
+	else
+	{
+		uint32		hashkey = _hash_get_indextuple_hashkey(itup);
+
+		itup_off = _hash_binsearch(page, hashkey);
+	}
 
 	if (PageAddItem(page, (Item) itup, itemsize, itup_off, false, false)
 		== InvalidOffsetNumber)

src/backend/access/hash/hashsort.c

@@ -145,7 +145,8 @@ _h_indexbuild(HSpool *hspool, Relation heapRel)
 		Assert(hashkey >= lasthashkey);
 #endif
 
-		_hash_doinsert(hspool->index, itup, heapRel);
+		/* the tuples are sorted by hashkey, so pass 'sorted' as true */
+		_hash_doinsert(hspool->index, itup, heapRel, true);
 
 		pgstat_progress_update_param(PROGRESS_CREATEIDX_TUPLES_DONE,
 									 ++tups_done);

src/include/access/hash.h

@@ -390,9 +390,11 @@ extern void hashadjustmembers(Oid opfamilyoid,
 /* private routines */
 
 /* hashinsert.c */
-extern void _hash_doinsert(Relation rel, IndexTuple itup, Relation heapRel);
+extern void _hash_doinsert(Relation rel, IndexTuple itup, Relation heapRel,
+						   bool sorted);
 extern OffsetNumber _hash_pgaddtup(Relation rel, Buffer buf,
-								   Size itemsize, IndexTuple itup);
+								   Size itemsize, IndexTuple itup,
+								   bool appendtup);
 extern void _hash_pgaddmultitup(Relation rel, Buffer buf, IndexTuple *itups,
 								OffsetNumber *itup_offsets, uint16 nitups);