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

3 files changed, 52 insertions, 0 deletions

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index fc22951e68f..f995a86e85c 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -3205,6 +3205,29 @@ reparameterize_path(PlannerInfo *root, Path *path,
 														 spath->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,
+									   apath->path.parallel_workers);
+			}
 		default:
 			break;
 	}
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index eafc933e8f5..1fafca0d2d3 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -5150,6 +5150,25 @@ 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 af6d6129c8f..3b55a1c75ea 100644
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -1666,6 +1666,16 @@ select * from
     select * from (select 3 as z offset 0) 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