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

@@ -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;
 }