Fix incorrect matching of subexpressions in outer-join plan nodes.

Previously we would re-use input subexpressions in all expression trees
attached to a Join plan node.  However, if it's an outer join and the
subexpression appears in the nullable-side input, this is potentially
incorrect for apparently-matching subexpressions that came from above
the outer join (ie, targetlist and qpqual expressions), because the
executor will treat the subexpression value as NULL when maybe it should
not be.

The case is fairly hard to hit because (a) you need a non-strict
subexpression (else NULL is correct), and (b) we don't usually compute
expressions in the outputs of non-toplevel plan nodes.  But we might do
so if the expressions are sort keys for a mergejoin, for example.

Probably in the long run we should make a more explicit distinction between
Vars appearing above and below an outer join, but that will be a major
planner redesign and not at all back-patchable.  For the moment, just hack
set_join_references so that it will not match any non-Var expressions
coming from nullable inputs to expressions that came from above the join.
(This is somewhat overkill, in that a strict expression could still be
matched, but it doesn't seem worth the effort to check that.)

Per report from Qingqing Zhou.  The added regression test case is based
on his example.

This has been broken for a very long time, so back-patch to all active
branches.

2 files changed, 68 insertions, 0 deletions

diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index c079f7f1251..bfb9e841ad9 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -2789,6 +2789,54 @@ explain (costs off)
 (6 rows)
 
 --
+-- test that quals attached to an outer join have correct semantics,
+-- specifically that they don't re-use expressions computed below the join;
+-- we force a mergejoin so that coalesce(b.q1, 1) appears as a join input
+--
+set enable_hashjoin to off;
+set enable_nestloop to off;
+explain (verbose, costs off)
+  select a.q2, b.q1
+    from int8_tbl a left join int8_tbl b on a.q2 = coalesce(b.q1, 1)
+    where coalesce(b.q1, 1) > 0;
+                      QUERY PLAN                       
+-------------------------------------------------------
+ Merge Left Join
+   Output: a.q2, b.q1
+   Merge Cond: (a.q2 = (COALESCE(b.q1, 1::bigint)))
+   Filter: (COALESCE(b.q1, 1::bigint) > 0)
+   ->  Sort
+         Output: a.q2
+         Sort Key: a.q2
+         ->  Seq Scan on public.int8_tbl a
+               Output: a.q2
+   ->  Sort
+         Output: b.q1, (COALESCE(b.q1, 1::bigint))
+         Sort Key: (COALESCE(b.q1, 1::bigint))
+         ->  Seq Scan on public.int8_tbl b
+               Output: b.q1, COALESCE(b.q1, 1::bigint)
+(14 rows)
+
+select a.q2, b.q1
+  from int8_tbl a left join int8_tbl b on a.q2 = coalesce(b.q1, 1)
+  where coalesce(b.q1, 1) > 0;
+        q2         |        q1        
+-------------------+------------------
+ -4567890123456789 |                 
+               123 |              123
+               123 |              123
+               456 |                 
+  4567890123456789 | 4567890123456789
+  4567890123456789 | 4567890123456789
+  4567890123456789 | 4567890123456789
+  4567890123456789 | 4567890123456789
+  4567890123456789 | 4567890123456789
+  4567890123456789 | 4567890123456789
+(10 rows)
+
+reset enable_hashjoin;
+reset enable_nestloop;
+--
 -- test join removal
 --
 begin;
diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index 56267912696..5a9f1df5506 100644
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -751,6 +751,26 @@ explain (costs off)
   select * from tenk1 a full join tenk1 b using(unique2) where unique2 = 42;
 
 --
+-- test that quals attached to an outer join have correct semantics,
+-- specifically that they don't re-use expressions computed below the join;
+-- we force a mergejoin so that coalesce(b.q1, 1) appears as a join input
+--
+
+set enable_hashjoin to off;
+set enable_nestloop to off;
+
+explain (verbose, costs off)
+  select a.q2, b.q1
+    from int8_tbl a left join int8_tbl b on a.q2 = coalesce(b.q1, 1)
+    where coalesce(b.q1, 1) > 0;
+select a.q2, b.q1
+  from int8_tbl a left join int8_tbl b on a.q2 = coalesce(b.q1, 1)
+  where coalesce(b.q1, 1) > 0;
+
+reset enable_hashjoin;
+reset enable_nestloop;
+
+--
 -- test join removal
 --