Thread: A problem about ParamPathInfo for an AppendPath

While trying the codes of MemoizePath I noticed that MemoizePath cannot
be generated if inner path is a union-all AppendPath, even if it is
parameterized.  And I found the problem is that for a union-all
AppendPath the ParamPathInfo is not created in the usual way.  Instead
it is created with get_appendrel_parampathinfo, which just creates a
struct with no ppi_clauses.  As a result, MemoizePath cannot find a
proper cache key to use.

This problem does not exist for AppendPath generated for a partitioned
table though, because in this case we always create a normal param_info
with ppi_clauses, for use in run-time pruning.

As an illustration, we can use the table 'prt' in sql/memoize.sql with
the settings below

set enable_hashjoin to off;
set enable_mergejoin to off;
set enable_seqscan to off;
set enable_material to off;

explain (costs off) select * from prt_p1 t1 join prt t2 on t1.a = t2.a;
                            QUERY PLAN
------------------------------------------------------------------
 Nested Loop
   ->  Index Only Scan using iprt_p1_a on prt_p1 t1
   ->  Memoize
         Cache Key: t1.a
         Cache Mode: logical
         ->  Append
               ->  Index Only Scan using iprt_p1_a on prt_p1 t2_1
                     Index Cond: (a = t1.a)
               ->  Index Only Scan using iprt_p2_a on prt_p2 t2_2
                     Index Cond: (a = t1.a)
(10 rows)

explain (costs off) select * from prt_p1 t1 join (select * from prt_p1 union all select * from prt_p2) t2 on t1.a = t2.a;
                      QUERY PLAN
-------------------------------------------------------
 Nested Loop
   ->  Index Only Scan using iprt_p1_a on prt_p1 t1
   ->  Append
         ->  Index Only Scan using iprt_p1_a on prt_p1
               Index Cond: (a = t1.a)
         ->  Index Only Scan using iprt_p2_a on prt_p2
               Index Cond: (a = t1.a)
(7 rows)

As we can see, MemoizePath can be generated for partitioned AppendPath
but not for union-all AppendPath.

For the fix I think we can relax the check in create_append_path and
always use get_baserel_parampathinfo if the parent is a baserel.

--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -1266,7 +1266,7 @@ create_append_path(PlannerInfo *root,
     * partition, and it's not necessary anyway in that case.  Must skip it if
     * we don't have "root", too.)
     */
-   if (root && rel->reloptkind == RELOPT_BASEREL && IS_PARTITIONED_REL(rel))
+   if (root && rel->reloptkind == RELOPT_BASEREL)
        pathnode->path.param_info = get_baserel_parampathinfo(root,
                                                              rel,
                                                              required_outer);

Thanks
Richard