From fbac1272b89b547dbaacd78bbe8da68e5493cbda Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 17 Mar 2004 22:24:58 +0000
Subject: [PATCH] During btree index build, sort equal-keyed tuples according
to their TID (heap position). This doesn't do anything to the validity of
the finished index, but by pretending to qsort() that there are no really
equal keys in the sort, we can avoid performance problems with qsort
implementations that have trouble with large numbers of equal keys. Patch
from Manfred Koizar.
---
src/backend/utils/sort/tuplesort.c | 26 ++++++++++++++++++++++++--
1 file changed, 24 insertions(+), 2 deletions(-)
diff --git a/src/backend/utils/sort/tuplesort.c b/src/backend/utils/sort/tuplesort.c
index 636fdedcdb4..549ca8c484e 100644
--- a/src/backend/utils/sort/tuplesort.c
+++ b/src/backend/utils/sort/tuplesort.c
@@ -78,7 +78,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/utils/sort/tuplesort.c,v 1.41 2004/02/03 17:34:03 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/sort/tuplesort.c,v 1.42 2004/03/17 22:24:58 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -2020,7 +2020,8 @@ comparetup_index(Tuplesortstate *state, const void *a, const void *b)
{
/*
* This is almost the same as _bt_tuplecompare(), but we need to keep
- * track of whether any null fields are present.
+ * track of whether any null fields are present. Also see the special
+ * treatment for equal keys at the end.
*/
IndexTuple tuple1 = (IndexTuple) a;
IndexTuple tuple2 = (IndexTuple) b;
@@ -2081,6 +2082,27 @@ comparetup_index(Tuplesortstate *state, const void *a, const void *b)
errmsg("could not create unique index"),
errdetail("Table contains duplicated values.")));
+ /*
+ * If key values are equal, we sort on ItemPointer. This does not
+ * affect validity of the finished index, but it offers cheap insurance
+ * against performance problems with bad qsort implementations that have
+ * trouble with large numbers of equal keys.
+ */
+ {
+ BlockNumber blk1 = ItemPointerGetBlockNumber(&tuple1->t_tid);
+ BlockNumber blk2 = ItemPointerGetBlockNumber(&tuple2->t_tid);
+
+ if (blk1 != blk2)
+ return (blk1 < blk2) ? -1 : 1;
+ }
+ {
+ OffsetNumber pos1 = ItemPointerGetOffsetNumber(&tuple1->t_tid);
+ OffsetNumber pos2 = ItemPointerGetOffsetNumber(&tuple2->t_tid);
+
+ if (pos1 != pos2)
+ return (pos1 < pos2) ? -1 : 1;
+ }
+
return 0;
}
--
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