Fix comment about ressortgrouprefs being unique in setop plans.

Author: Richard Guo, Tom Lane
Discussion: https://www.postgresql.org/message-id/CAMbWs49rAfFS-yd7=QxtDUrZDFfRBGy4rGBJNyGDH7=CLipFPg@mail.gmail.com

diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c
index fc3709510d2c673b5a299fc52db745c1f071cc7a..4bb68ac90e7d6223936b548e25537567e9903aba 100644 (file)
--- a/src/backend/optimizer/plan/setrefs.c
+++ b/src/backend/optimizer/plan/setrefs.c
@@ -2936,7 +2936,14 @@ search_indexed_tlist_for_sortgroupref(Expr *node,
    {
        TargetEntry *tle = (TargetEntry *) lfirst(lc);
 
-       /* The equal() check should be redundant, but let's be paranoid */
+       /*
+        * Usually the equal() check is redundant, but in setop plans it may
+        * not be, since prepunion.c assigns ressortgroupref equal to the
+        * column resno without regard to whether that matches the topmost
+        * level's sortgrouprefs and without regard to whether any implicit
+        * coercions are added in the setop tree.  We might have to clean that
+        * up someday; but for now, just ignore any false matches.
+        */
        if (tle->ressortgroupref == sortgroupref &&
            equal(node, tle->expr))
        {