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 97f0ec711462068a69dd4779aae009711e29d3cd..b09fe672664dadf586cd2740af88380ecb1b0878 100644 (file)
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -2845,8 +2845,10 @@ 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)
@@ -2866,7 +2868,19 @@ 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 e9ae5e8383a25a5a312c9f1b25cbe552fb92c614..ff295916db75c24898ab8fb640682f76d6e7bcb2 100644 (file)
--- a/src/test/regress/expected/subselect.out
+++ b/src/test/regress/expected/subselect.out
@@ -980,6 +980,59 @@ 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 9f5513a21837dbe00f753d181fc6487238bd5dbf..55c3df32f54371461c726abc52f55fc0d8b15107 100644 (file)
--- a/src/test/regress/sql/subselect.sql
+++ b/src/test/regress/sql/subselect.sql
@@ -526,6 +526,32 @@ 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