Disable physical tlist if any Var would need multiple sortgroupref labels.

As part of upper planner pathification (commit 3fc6e2d7f5b652b4) I redid
createplan.c's approach to the physical-tlist optimization, in which scan
nodes are allowed to return exactly the underlying table's columns so as
to save doing a projection step at runtime.  The logic was intentionally
more aggressive than before about applying the optimization, which is
generally a good thing, but Andres Freund found a case in which it got
too aggressive.  Namely, if any column is referenced more than once in
the parent plan node's sorting or grouping column list, we can't optimize
because then that column would need to have more than one ressortgroupref
label, and we only have space for one.

Add logic to detect this situation in use_physical_tlist(), and also add
some error checking in apply_pathtarget_labeling_to_tlist(), which this
example proves was being overly cavalier about whether what it was doing
made any sense.

The added test case exposes the problem only because we do not eliminate
duplicate grouping keys.  That might be something to fix someday, but it
doesn't seem like appropriate post-beta work.

Report: <[email protected]>

1 files changed, 24 insertions, 0 deletions

diff --git a/src/test/regress/expected/select_distinct.out b/src/test/regress/expected/select_distinct.out
index 38107a04133..f3696c6d1de 100644
--- a/src/test/regress/expected/select_distinct.out
+++ b/src/test/regress/expected/select_distinct.out
@@ -125,6 +125,30 @@ SELECT DISTINCT p.age FROM person* p ORDER BY age using >;
 (20 rows)
 
 --
+-- Check mentioning same column more than once
+--
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT count(*) FROM
+  (SELECT DISTINCT two, four, two FROM tenk1) ss;
+                       QUERY PLAN                       
+--------------------------------------------------------
+ Aggregate
+   Output: count(*)
+   ->  HashAggregate
+         Output: tenk1.two, tenk1.four, tenk1.two
+         Group Key: tenk1.two, tenk1.four, tenk1.two
+         ->  Seq Scan on public.tenk1
+               Output: tenk1.two, tenk1.four, tenk1.two
+(7 rows)
+
+SELECT count(*) FROM
+  (SELECT DISTINCT two, four, two FROM tenk1) ss;
+ count 
+-------
+     4
+(1 row)
+
+--
 -- Also, some tests of IS DISTINCT FROM, which doesn't quite deserve its
 -- very own regression file.
 --