Enable use of Memoize atop an Append that came from UNION ALL.

create_append_path() would only apply get_baserel_parampathinfo
when the path is for a partitioned table, but it's also potentially
useful for paths for UNION ALL appendrels.  Specifically, that
supports building a Memoize path atop this one.

While we're in the vicinity, delete some dead code in
create_merge_append_plan(): there's no need for it to support
parameterized MergeAppend paths, and it doesn't look like that
is going to change anytime soon.  It'll be easy enough to undo
this when/if it becomes useful.

Richard Guo

Discussion: https://postgr.es/m/CAMbWs4_ABSu4PWG2rE1q10tJugEXHWgru3U8dAgkoFvgrb6aEA@mail.gmail.com

src/backend/optimizer/plan/createplan.c

@@ -1531,16 +1531,8 @@ create_merge_append_plan(PlannerInfo *root, MergeAppendPath *best_path,
 
 		prunequal = extract_actual_clauses(rel->baserestrictinfo, false);
 
-		if (best_path->path.param_info)
-		{
-			List	   *prmquals = best_path->path.param_info->ppi_clauses;
-
-			prmquals = extract_actual_clauses(prmquals, false);
-			prmquals = (List *) replace_nestloop_params(root,
-														(Node *) prmquals);
-
-			prunequal = list_concat(prunequal, prmquals);
-		}
+		/* We don't currently generate any parameterized MergeAppend paths */
+		Assert(best_path->path.param_info == NULL);
 
 		if (prunequal != NIL)
 			node->part_prune_index = make_partition_pruneinfo(root, rel,

src/backend/optimizer/util/pathnode.c

@@ -1256,15 +1256,17 @@ create_append_path(PlannerInfo *root,
 	pathnode->path.pathtarget = rel->reltarget;
 
 	/*
-	 * When generating an Append path for a partitioned table, there may be
-	 * parameterized quals that are useful for run-time pruning.  Hence,
-	 * compute path.param_info the same way as for any other baserel, so that
-	 * such quals will be available for make_partition_pruneinfo().  (This
-	 * would not work right for a non-baserel, ie a scan on a non-leaf child
-	 * partition, and it's not necessary anyway in that case.  Must skip it if
-	 * we don't have "root", too.)
+	 * If this is for a baserel (not a join or non-leaf partition), we prefer
+	 * to apply get_baserel_parampathinfo to construct a full ParamPathInfo
+	 * for the path.  This supports building a Memoize path atop this path,
+	 * and if this is a partitioned table the info may be useful for run-time
+	 * pruning (cf make_partition_pruneinfo()).
+	 *
+	 * However, if we don't have "root" then that won't work and we fall back
+	 * on the simpler get_appendrel_parampathinfo.  There's no point in doing
+	 * the more expensive thing for a dummy path, either.
 	 */
-	if (root && rel->reloptkind == RELOPT_BASEREL && IS_PARTITIONED_REL(rel))
+	if (rel->reloptkind == RELOPT_BASEREL && root && subpaths != NIL)
 		pathnode->path.param_info = get_baserel_parampathinfo(root,
 															  rel,
 															  required_outer);

src/test/regress/expected/memoize.out

@@ -236,6 +236,30 @@ SELECT * FROM prt t1 INNER JOIN prt t2 ON t1.a = t2.a;', false);
                      Heap Fetches: N
 (21 rows)
 
+-- Ensure memoize works with parameterized union-all Append path
+SET enable_partitionwise_join TO off;
+SELECT explain_memoize('
+SELECT * FROM prt_p1 t1 INNER JOIN
+(SELECT * FROM prt_p1 UNION ALL SELECT * FROM prt_p2) t2
+ON t1.a = t2.a;', false);
+                                   explain_memoize                                   
+-------------------------------------------------------------------------------------
+ Nested Loop (actual rows=16 loops=N)
+   ->  Index Only Scan using iprt_p1_a on prt_p1 t1 (actual rows=4 loops=N)
+         Heap Fetches: N
+   ->  Memoize (actual rows=4 loops=N)
+         Cache Key: t1.a
+         Cache Mode: logical
+         Hits: 3  Misses: 1  Evictions: Zero  Overflows: 0  Memory Usage: NkB
+         ->  Append (actual rows=4 loops=N)
+               ->  Index Only Scan using iprt_p1_a on prt_p1 (actual rows=4 loops=N)
+                     Index Cond: (a = t1.a)
+                     Heap Fetches: N
+               ->  Index Only Scan using iprt_p2_a on prt_p2 (actual rows=0 loops=N)
+                     Index Cond: (a = t1.a)
+                     Heap Fetches: N
+(14 rows)
+
 DROP TABLE prt;
 RESET enable_partitionwise_join;
 -- Exercise Memoize code that flushes the cache when a parameter changes which

src/test/regress/sql/memoize.sql

@@ -121,6 +121,14 @@ ANALYZE prt;
 SELECT explain_memoize('
 SELECT * FROM prt t1 INNER JOIN prt t2 ON t1.a = t2.a;', false);
 
+-- Ensure memoize works with parameterized union-all Append path
+SET enable_partitionwise_join TO off;
+
+SELECT explain_memoize('
+SELECT * FROM prt_p1 t1 INNER JOIN
+(SELECT * FROM prt_p1 UNION ALL SELECT * FROM prt_p2) t2
+ON t1.a = t2.a;', false);
+
 DROP TABLE prt;
 
 RESET enable_partitionwise_join;