From ee63981c1fe26299162b9c7f1218d7e3ef802409 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 23 Sep 2010 15:34:56 -0400
Subject: [PATCH] Avoid sharing subpath list structure when flattening nested
 AppendRels.

In some situations the original coding led to corrupting the child AppendRel's
subpaths list, effectively adding other members of the parent's list to it.
This was usually masked because we never made any further use of the child's
list, but given the right combination of circumstances, we could do so.  The
visible symptom would be a relation getting scanned twice, as in bug #5673
from David Schmitt.

Backpatch to 8.2, which is as far back as the risky coding appears.  The
example submitted by David only fails in 8.4 and later, but I'm not convinced
that there aren't any even-more-obscure cases where 8.2 and 8.3 would fail.
---
 src/backend/optimizer/path/allpaths.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 2d86da3835f..b7cf0b815b7 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -425,7 +425,7 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 		childpath = childrel->cheapest_total_path;
 		if (IsA(childpath, AppendPath))
 			subpaths = list_concat(subpaths,
-								   ((AppendPath *) childpath)->subpaths);
+							list_copy(((AppendPath *) childpath)->subpaths));
 		else
 			subpaths = lappend(subpaths, childpath);
 
-- 
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