Do not allow Unique nodes to be scanned backwards. The code claimed that it

would work, but in fact it didn't return the same rows when moving backwards
as when moving forwards.  This would have no visible effect in a DISTINCT
query (at least assuming the column datatypes use a strong definition of
equality), but it gave entirely wrong answers for DISTINCT ON queries.

diff --git a/src/backend/executor/execAmi.c b/src/backend/executor/execAmi.c
index 86d09345106bb3e861c7ee27ec955c8c9e5f29f3..a0b58852c67f5b9e3b0144daa3864791b52c1673 100644 (file)
--- a/src/backend/executor/execAmi.c
+++ b/src/backend/executor/execAmi.c
@@ -338,9 +338,6 @@ ExecSupportsBackwardScan(Plan *node)
                case T_Sort:
                        return true;
 
-               case T_Unique:
-                       return ExecSupportsBackwardScan(outerPlan(node));
-
                case T_Limit:
                        return ExecSupportsBackwardScan(outerPlan(node));