Cope with lateral references in the quals of a subquery RTE.

The qual pushdown logic assumed that all Vars in a restriction clause
must be Vars referencing subquery outputs; but since we introduced
LATERAL, it's possible for such a Var to be a lateral reference instead.
This led to an assertion failure in debug builds.  In a non-debug
build, there might be no ill effects (if qual_is_pushdown_safe decided
the qual was unsafe anyway), or we could get failures later due to
construction of an invalid plan.  I've not gone to much length to
characterize the possible failures, but at least segfaults in the
executor have been observed.

Given that this has been busted since 9.3 and it took this long for
anybody to notice, I judge that the case isn't worth going to great
lengths to optimize.  Hence, fix by just teaching qual_is_pushdown_safe
that such quals are unsafe to push down, matching the previous behavior
when it accidentally didn't fail.

Per report from Tom Ellis.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/20200713175124.GQ8220@cloudinit-builder

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index b601cc6a11b75f31e668747d315b3d029f671822..0c6a348f0ea60288a20997c51eb301264427fcea 100644 (file)
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -3154,8 +3154,10 @@ qual_is_pushdown_safe(Query *subquery, Index rti, Node *qual,
    Assert(!contain_window_function(qual));
 
    /*
-    * Examine all Vars used in clause; since it's a restriction clause, all
-    * such Vars must refer to subselect output columns.
+    * Examine all Vars used in clause.  Since it's a restriction clause, all
+    * such Vars must refer to subselect output columns ... unless this is
+    * part of a LATERAL subquery, in which case there could be lateral
+    * references.
     */
    vars = pull_var_clause(qual, PVC_INCLUDE_PLACEHOLDERS);
    foreach(vl, vars)
@@ -3175,7 +3177,19 @@ qual_is_pushdown_safe(Query *subquery, Index rti, Node *qual,
            break;
        }
 
-       Assert(var->varno == rti);
+       /*
+        * Punt if we find any lateral references.  It would be safe to push
+        * these down, but we'd have to convert them into outer references,
+        * which subquery_push_qual lacks the infrastructure to do.  The case
+        * arises so seldom that it doesn't seem worth working hard on.
+        */
+       if (var->varno != rti)
+       {
+           safe = false;
+           break;
+       }
+
+       /* Subqueries have no system columns */
        Assert(var->varattno >= 0);
 
        /* Check point 4 */

diff --git a/src/test/regress/expected/subselect.out b/src/test/regress/expected/subselect.out
index 181a792fcf11ba19169e0d268d57ea2a4aa6fd6b..48e1572afa4bc857831957add182a6b5e2aa2cbe 100644 (file)
--- a/src/test/regress/expected/subselect.out
+++ b/src/test/regress/expected/subselect.out
@@ -1127,6 +1127,59 @@ from int4_tbl;
  (4,5,6.0)
 (5 rows)
 
+--
+-- Check for sane handling of a lateral reference in a subquery's quals
+-- (most of the complication here is to prevent the test case from being
+-- flattened too much)
+--
+explain (verbose, costs off)
+select * from
+    int4_tbl i4,
+    lateral (
+        select i4.f1 > 1 as b, 1 as id
+        from (select random() order by 1) as t1
+      union all
+        select true as b, 2 as id
+    ) as t2
+where b and f1 >= 0;
+                 QUERY PLAN                 
+--------------------------------------------
+ Nested Loop
+   Output: i4.f1, ((i4.f1 > 1)), (1)
+   ->  Seq Scan on public.int4_tbl i4
+         Output: i4.f1
+         Filter: (i4.f1 >= 0)
+   ->  Append
+         ->  Subquery Scan on t1
+               Output: (i4.f1 > 1), 1
+               Filter: (i4.f1 > 1)
+               ->  Sort
+                     Output: (random())
+                     Sort Key: (random())
+                     ->  Result
+                           Output: random()
+         ->  Result
+               Output: true, 2
+(16 rows)
+
+select * from
+    int4_tbl i4,
+    lateral (
+        select i4.f1 > 1 as b, 1 as id
+        from (select random() order by 1) as t1
+      union all
+        select true as b, 2 as id
+    ) as t2
+where b and f1 >= 0;
+     f1     | b | id 
+------------+---+----
+          0 | t |  2
+     123456 | t |  1
+     123456 | t |  2
+ 2147483647 | t |  1
+ 2147483647 | t |  2
+(5 rows)
+
 --
 -- Check that volatile quals aren't pushed down past a DISTINCT:
 -- nextval() should not be called more than the nominal number of times

diff --git a/src/test/regress/sql/subselect.sql b/src/test/regress/sql/subselect.sql
index 527d512b6907c93f5078e609995ac90d0571196f..9909ee28963f63426f82ab417e50d6cfc41ed770 100644 (file)
--- a/src/test/regress/sql/subselect.sql
+++ b/src/test/regress/sql/subselect.sql
@@ -601,6 +601,32 @@ select (select q from
          ) q )
 from int4_tbl;
 
+--
+-- Check for sane handling of a lateral reference in a subquery's quals
+-- (most of the complication here is to prevent the test case from being
+-- flattened too much)
+--
+explain (verbose, costs off)
+select * from
+    int4_tbl i4,
+    lateral (
+        select i4.f1 > 1 as b, 1 as id
+        from (select random() order by 1) as t1
+      union all
+        select true as b, 2 as id
+    ) as t2
+where b and f1 >= 0;
+
+select * from
+    int4_tbl i4,
+    lateral (
+        select i4.f1 > 1 as b, 1 as id
+        from (select random() order by 1) as t1
+      union all
+        select true as b, 2 as id
+    ) as t2
+where b and f1 >= 0;
+
 --
 -- Check that volatile quals aren't pushed down past a DISTINCT:
 -- nextval() should not be called more than the nominal number of times