Teach reparameterize_path() to handle AppendPaths.

If we're inside a lateral subquery, there may be no unparameterized paths
for a particular child relation of an appendrel, in which case we *must*
be able to create similarly-parameterized paths for each other child
relation, else the planner will fail with "could not devise a query plan
for the given query".  This means that there are situations where we'd
better be able to reparameterize at least one path for each child.

This calls into question the assumption in reparameterize_path() that
it can just punt if it feels like it.  However, the only case that is
known broken right now is where the child is itself an appendrel so that
all its paths are AppendPaths.  (I think possibly I disregarded that in
the original coding on the theory that nested appendrels would get folded
together --- but that only happens *after* reparameterize_path(), so it's
not excused from handling a child AppendPath.)  Given that this code's been
like this since 9.3 when LATERAL was introduced, it seems likely we'd have
heard of other cases by now if there were a larger problem.

Per report from Elvis Pranskevichus.  Back-patch to 9.3.

Discussion: https://postgr.es/m/5981018.zdth1YWmNy@hammer.magicstack.net

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index c6c12465c2616cd1239d066da558d227f4504ca5..bcaa62919c6c7e885c901327df080383c354b72e 100644 (file)
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -2075,6 +2075,28 @@ reparameterize_path(PlannerInfo *root, Path *path,
        case T_SubqueryScan:
            return create_subqueryscan_path(root, rel, path->pathkeys,
                                            required_outer);
+       case T_Append:
+           {
+               AppendPath *apath = (AppendPath *) path;
+               List       *childpaths = NIL;
+               ListCell   *lc;
+
+               /* Reparameterize the children */
+               foreach(lc, apath->subpaths)
+               {
+                   Path       *spath = (Path *) lfirst(lc);
+
+                   spath = reparameterize_path(root, spath,
+                                               required_outer,
+                                               loop_count);
+                   if (spath == NULL)
+                       return NULL;
+                   childpaths = lappend(childpaths, spath);
+               }
+               return (Path *)
+                   create_append_path(rel, childpaths,
+                                      required_outer);
+           }
        default:
            break;
    }

diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index bb599a6e4eaa8c4b10877282d5021f33107b8ebd..f2e51612c5ecad898230fb54e95d5d6cb845b5d2 100644 (file)
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -4880,6 +4880,25 @@ select * from
          Output: 3
 (11 rows)
 
+-- check handling of nested appendrels inside LATERAL
+select * from
+  ((select 2 as v) union all (select 3 as v)) as q1
+  cross join lateral
+  ((select * from
+      ((select 4 as v) union all (select 5 as v)) as q3)
+   union all
+   (select q1.v)
+  ) as q2;
+ v | v 
+---+---
+ 2 | 4
+ 2 | 5
+ 2 | 2
+ 3 | 4
+ 3 | 5
+ 3 | 3
+(6 rows)
+
 -- check we don't try to do a unique-ified semijoin with LATERAL
 explain (verbose, costs off)
 select * from

diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index 40dd87963071d03a7b7fb6030c7da557a40d8d11..1bbbb9cca86e8d92e3d4d85d519bb5102d319bdc 100644 (file)
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -1553,6 +1553,16 @@ select * from
     select * from (select 3 as z) z where z.z = x.x
   ) zz on zz.z = y.y;
 
+-- check handling of nested appendrels inside LATERAL
+select * from
+  ((select 2 as v) union all (select 3 as v)) as q1
+  cross join lateral
+  ((select * from
+      ((select 4 as v) union all (select 5 as v)) as q3)
+   union all
+   (select q1.v)
+  ) as q2;
+
 -- check we don't try to do a unique-ified semijoin with LATERAL
 explain (verbose, costs off)
 select * from