From b67ad046e6c37cbe0eebc5745323ed9864192f52 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 16 Mar 2012 14:07:21 -0400
Subject: [PATCH] Improve commentary in match_pathkeys_to_index().
For a little while there I thought match_pathkeys_to_index() was broken
because it wasn't trying to match index columns to pathkeys in order.
Actually that's correct, because GiST can support ordering operators
on any random collection of index columns, but it sure needs a comment.
---
src/backend/optimizer/path/indxpath.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index 89b42da6b46..e23cc5eb7b2 100644
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -2174,6 +2174,13 @@ match_pathkeys_to_index(IndexOptInfo *index, List *pathkeys,
if (!bms_equal(member->em_relids, index->rel->relids))
continue;
+ /*
+ * We allow any column of the index to match each pathkey; they
+ * don't have to match left-to-right as you might expect. This
+ * is correct for GiST, which is the sole existing AM supporting
+ * amcanorderbyop. We might need different logic in future for
+ * other implementations.
+ */
for (indexcol = 0; indexcol < index->ncolumns; indexcol++)
{
Expr *expr;
--
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