Add child_append_relid_sets to ElidedNode; use in pg_plan_advice.

Commit 0d4391b265 taught the planner
to store information about the RTIs of SubqueryScan, Append, and
MergeAppend node elided from the plan using a new ElidedNode data
structure; an hour later, 7358abcc60
taught to save the RTIs of an Append node into another Append
node using a new child_append_relid_sets field.  Regrettably,
despite the fact that both commits were written by me and the
fact that both were part of the same patch set, the latter commit
failed to deal with the case where an Append node first acquires
child_append_relid_sets != NIL and then gets elided. I believe that
I thought that was impossible, but the included test case shows that
it isn't.

The proper fix involves updating the definition of ElidedNode to
carry child_append_relid_sets, so this commit does that, breaking
ABI compatibility. That's fine for the master branch, but it's a
bit unfortunate to be doing do that so close to the planned ship
date for v19.  However, shipping v19 without fixing this appears
(at least to me) to be even more unfortunate.

Backpatch-through: 19
Discussion: http://postgr.es/m/CA+TgmoYYHC4i-Gpzc9-JYHqih6pOK+6LTYFiNR-PioXG1nKkOQ@mail.gmail.com

contrib/pg_overexplain/expected/pg_overexplain.out

@@ -592,6 +592,7 @@ SELECT * FROM vegetables WHERE genus = 'daucus';
    Scan RTI: 2
    Elided Node Type: Append
    Elided Node RTIs: 1
+   Elided Node Child Append RTIs: none
  RTI 1 (relation, inherited, in-from-clause):
    Eref: vegetables (id, name, genus)
    Relation: vegetables
@@ -605,7 +606,7 @@ SELECT * FROM vegetables WHERE genus = 'daucus';
    Relation Kind: relation
    Relation Lock Mode: AccessShareLock
  Unprunable RTIs: 1 2
-(18 rows)
+(19 rows)
 
 -- Also test a case that involves a write.
 EXPLAIN (RANGE_TABLE, COSTS OFF)
@@ -641,6 +642,7 @@ SELECT * FROM vegetables v,
          Scan RTI: 6
          Elided Node Type: Append
          Elided Node RTIs: 5
+         Elided Node Child Append RTIs: none
          Elided Node Type: SubqueryScan
          Elided Node RTIs: 2
    ->  Append
@@ -687,7 +689,7 @@ SELECT * FROM vegetables v,
    Relation Kind: relation
    Relation Lock Mode: AccessShareLock
  Unprunable RTIs: 1 3 4 5 6
-(52 rows)
+(53 rows)
 
 -- should show "Subplan: unnamed_subquery"
 EXPLAIN (RANGE_TABLE, COSTS OFF)
@@ -701,6 +703,7 @@ SELECT * FROM vegetables v,
          Scan RTI: 6
          Elided Node Type: Append
          Elided Node RTIs: 5
+         Elided Node Child Append RTIs: none
          Elided Node Type: SubqueryScan
          Elided Node RTIs: 2
    ->  Append
@@ -746,7 +749,7 @@ SELECT * FROM vegetables v,
    Relation Kind: relation
    Relation Lock Mode: AccessShareLock
  Unprunable RTIs: 1 3 4 5 6
-(51 rows)
+(52 rows)
 
 -- test display of child append RTIs
 EXPLAIN (RANGE_TABLE, COSTS OFF)

contrib/pg_overexplain/pg_overexplain.c

@@ -297,6 +297,9 @@ overexplain_per_node_hook(PlanState *planstate, List *ancestors,
 			ExplainOpenGroup("Elided Node", NULL, true, es);
 			ExplainPropertyText("Elided Node Type", elidednodetag, es);
 			overexplain_bitmapset("Elided Node RTIs", n->relids, es);
+			if (n->elided_type == T_Append || n->elided_type == T_MergeAppend)
+				overexplain_bitmapset_list("Elided Node Child Append RTIs",
+										   n->child_append_relid_sets, es);
 			ExplainCloseGroup("Elided Node", NULL, true, es);
 		}
 		if (opened_elided_nodes)

contrib/pg_plan_advice/expected/partitionwise.out

@@ -476,3 +476,16 @@ SELECT * FROM mllpt WHERE a = 1 UNION ALL SELECT * FROM mllpt;
     mllpt/public.mllpt_a1_b2@unnamed_subquery mllpt/public.mllpt_a1_b3@unnamed_subquery)
 (27 rows)
 
+-- Same, but prune down to a single leaf so the Append is elided.
+EXPLAIN (PLAN_ADVICE, COSTS OFF)
+SELECT * FROM mllpt WHERE a = 1 AND b = 1;
+                  QUERY PLAN                  
+----------------------------------------------
+ Seq Scan on mllpt_a1_b1 mllpt
+   Filter: ((a = 1) AND (b = 1))
+ Generated Plan Advice:
+   SEQ_SCAN(mllpt/public.mllpt_a1_b1)
+   PARTITIONWISE(mllpt/public.mllpt_a1 mllpt)
+   NO_GATHER(mllpt/public.mllpt_a1_b1)
+(6 rows)
+

contrib/pg_plan_advice/pgpa_scan.c

@@ -83,6 +83,9 @@ pgpa_build_scan(pgpa_plan_walker_context *walker, Plan *plan,
 		else
 			strategy = PGPA_SCAN_ORDINARY;
 
+		/* Be sure to account for pulled-up scans, as for a live Append. */
+		child_append_relid_sets = elided_node->child_append_relid_sets;
+
 		/* Join RTIs can be present, but advice never refers to them. */
 		relids = pgpa_filter_out_join_relids(relids, walker->pstmt->rtable);
 	}

contrib/pg_plan_advice/sql/partitionwise.sql

@@ -123,3 +123,7 @@ COMMIT;
 -- We should get PARTITIONWISE advice for all unpruned partition tables.
 EXPLAIN (PLAN_ADVICE, COSTS OFF)
 SELECT * FROM mllpt WHERE a = 1 UNION ALL SELECT * FROM mllpt;
+
+-- Same, but prune down to a single leaf so the Append is elided.
+EXPLAIN (PLAN_ADVICE, COSTS OFF)
+SELECT * FROM mllpt WHERE a = 1 AND b = 1;

src/backend/optimizer/plan/setrefs.c

@@ -214,7 +214,8 @@ static List *set_windowagg_runcondition_references(PlannerInfo *root,
 												   Plan *plan);
 
 static void record_elided_node(PlannerGlobal *glob, int plan_node_id,
-							   NodeTag elided_type, Bitmapset *relids);
+							   NodeTag elided_type, Bitmapset *relids,
+							   List *child_append_relid_sets);
 
 
 /*****************************************************************************
@@ -1480,7 +1481,8 @@ set_subqueryscan_references(PlannerInfo *root,
 		/* Remember that we removed a SubqueryScan */
 		scanrelid = plan->scan.scanrelid + rtoffset;
 		record_elided_node(root->glob, plan->subplan->plan_node_id,
-						   T_SubqueryScan, bms_make_singleton(scanrelid));
+						   T_SubqueryScan, bms_make_singleton(scanrelid),
+						   NIL);
 	}
 	else
 	{
@@ -1915,7 +1917,9 @@ set_append_references(PlannerInfo *root,
 
 			/* Remember that we removed an Append */
 			record_elided_node(root->glob, p->plan_node_id, T_Append,
-							   offset_relid_set(aplan->apprelids, rtoffset));
+							   offset_relid_set(aplan->apprelids, rtoffset),
+							   offset_relid_set_list(aplan->child_append_relid_sets,
+													 rtoffset));
 
 			return result;
 		}
@@ -1995,7 +1999,9 @@ set_mergeappend_references(PlannerInfo *root,
 
 			/* Remember that we removed a MergeAppend */
 			record_elided_node(root->glob, p->plan_node_id, T_MergeAppend,
-							   offset_relid_set(mplan->apprelids, rtoffset));
+							   offset_relid_set(mplan->apprelids, rtoffset),
+							   offset_relid_set_list(mplan->child_append_relid_sets,
+													 rtoffset));
 
 			return result;
 		}
@@ -3873,13 +3879,15 @@ extract_query_dependencies_walker(Node *node, PlannerInfo *context)
  */
 static void
 record_elided_node(PlannerGlobal *glob, int plan_node_id,
-				   NodeTag elided_type, Bitmapset *relids)
+				   NodeTag elided_type, Bitmapset *relids,
+				   List *child_append_relid_sets)
 {
 	ElidedNode *n = makeNode(ElidedNode);
 
 	n->plan_node_id = plan_node_id;
 	n->elided_type = elided_type;
 	n->relids = relids;
+	n->child_append_relid_sets = child_append_relid_sets;
 
 	glob->elidedNodes = lappend(glob->elidedNodes, n);
 }

src/include/nodes/plannodes.h

@@ -1862,6 +1862,9 @@ typedef struct SubPlanRTInfo
  *
  * plan_node_id is that of the surviving plan node, the sole child of the
  * one which was elided.
+ *
+ * For an elided Append or MergeAppend, child_append_relid_sets is the
+ * child_append_relid_sets value from the removed node; otherwise, it is NIL.
  */
 typedef struct ElidedNode
 {
@@ -1869,6 +1872,7 @@ typedef struct ElidedNode
 	int			plan_node_id;
 	NodeTag		elided_type;
 	Bitmapset  *relids;
+	List	   *child_append_relid_sets;
 } ElidedNode;
 
 #endif							/* PLANNODES_H */