Avoid mislabeling of lateral references when pulling up a subquery.

If we are pulling up a subquery that's under an outer join, and
the subquery's target list contains a strict expression that uses
both a subquery variable and a lateral-reference variable, it's okay
to pull up the expression without wrapping it in a PlaceHolderVar.
That's safe because if the subquery variable is forced to NULL
by the outer join, the expression result will come out as NULL too,
so we don't have to force that outcome by evaluating the expression
below the outer join.  It'd be correct to wrap in a PHV, but that can
lead to very significantly worse plans, since we'd then have to use
a nestloop plan to pass down the lateral reference to where the
expression will be evaluated.

However, when we do that, we should not mark the lateral reference
variable as being nulled by the outer join, because it isn't after
we pull up the expression in this way.  So the marking logic added
by cb8e50a4a was incorrect in this detail, leading to "wrong
varnullingrels" errors from the consistency-checking logic in
setrefs.c.  It seems to be sufficient to just not mark lateral
references at all in this case.  (I have a nagging feeling that more
complexity may be needed in cases where there are several levels of
outer join, but some attempts to break it with that didn't succeed.)

Per report from Bertrand Mamasam.  Back-patch to v16, as the previous
patch was.

Discussion: https://postgr.es/m/CACZ67_UA_EVrqiFXJu9XK50baEpH=ofEPJswa2kFxg6xuSw-ww@mail.gmail.com

2 files changed, 63 insertions, 0 deletions

diff --git a/src/test/regress/expected/subselect.out b/src/test/regress/expected/subselect.out
index 2d35de3fad6..da219aba4c4 100644
--- a/src/test/regress/expected/subselect.out
+++ b/src/test/regress/expected/subselect.out
@@ -1750,6 +1750,53 @@ where tname = 'tenk1' and attnum = 1;
  tenk1 | unique1
 (1 row)
 
+-- Check behavior when there's a lateral reference in the output expression
+explain (verbose, costs off)
+select t1.ten, sum(x) from
+  tenk1 t1 left join lateral (
+    select t1.ten + t2.ten as x, t2.fivethous from tenk1 t2
+  ) ss on t1.unique1 = ss.fivethous
+group by t1.ten
+order by t1.ten;
+                                                                                                    QUERY PLAN                                                                                                     
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Sort
+   Output: t1.ten, (sum((t1.ten + t2.ten)))
+   Sort Key: t1.ten
+   ->  HashAggregate
+         Output: t1.ten, sum((t1.ten + t2.ten))
+         Group Key: t1.ten
+         ->  Hash Right Join
+               Output: t1.ten, t2.ten
+               Hash Cond: (t2.fivethous = t1.unique1)
+               ->  Seq Scan on public.tenk1 t2
+                     Output: t2.unique1, t2.unique2, t2.two, t2.four, t2.ten, t2.twenty, t2.hundred, t2.thousand, t2.twothousand, t2.fivethous, t2.tenthous, t2.odd, t2.even, t2.stringu1, t2.stringu2, t2.string4
+               ->  Hash
+                     Output: t1.ten, t1.unique1
+                     ->  Seq Scan on public.tenk1 t1
+                           Output: t1.ten, t1.unique1
+(15 rows)
+
+select t1.ten, sum(x) from
+  tenk1 t1 left join lateral (
+    select t1.ten + t2.ten as x, t2.fivethous from tenk1 t2
+  ) ss on t1.unique1 = ss.fivethous
+group by t1.ten
+order by t1.ten;
+ ten |  sum  
+-----+-------
+   0 |     0
+   1 |  2000
+   2 |  4000
+   3 |  6000
+   4 |  8000
+   5 | 10000
+   6 | 12000
+   7 | 14000
+   8 | 16000
+   9 | 18000
+(10 rows)
+
 --
 -- Tests for CTE inlining behavior
 --
diff --git a/src/test/regress/sql/subselect.sql b/src/test/regress/sql/subselect.sql
index af6e157aca0..1815a5e6642 100644
--- a/src/test/regress/sql/subselect.sql
+++ b/src/test/regress/sql/subselect.sql
@@ -908,6 +908,22 @@ select relname::information_schema.sql_identifier as tname, * from
   right join pg_attribute a on a.attrelid = ss2.oid
 where tname = 'tenk1' and attnum = 1;
 
+-- Check behavior when there's a lateral reference in the output expression
+explain (verbose, costs off)
+select t1.ten, sum(x) from
+  tenk1 t1 left join lateral (
+    select t1.ten + t2.ten as x, t2.fivethous from tenk1 t2
+  ) ss on t1.unique1 = ss.fivethous
+group by t1.ten
+order by t1.ten;
+
+select t1.ten, sum(x) from
+  tenk1 t1 left join lateral (
+    select t1.ten + t2.ten as x, t2.fivethous from tenk1 t2
+  ) ss on t1.unique1 = ss.fivethous
+group by t1.ten
+order by t1.ten;
+
 --
 -- Tests for CTE inlining behavior
 --