Speed up planning when partitions can be pruned at plan time.

Previously, the planner created RangeTblEntry and RelOptInfo structs
for every partition of a partitioned table, even though many of them
might later be deemed uninteresting thanks to partition pruning logic.
This incurred significant overhead when there are many partitions.
Arrange to postpone creation of these data structures until after
we've processed the query enough to identify restriction quals for
the partitioned table, and then apply partition pruning before not
after creation of each partition's data structures.  In this way
we need not open the partition relations at all for partitions that
the planner has no real interest in.

For queries that can be proven at plan time to access only a small
number of partitions, this patch improves the practical maximum
number of partitions from under 100 to perhaps a few thousand.

Amit Langote, reviewed at various times by Dilip Kumar, Jesper Pedersen,
Yoshikazu Imai, and David Rowley

Discussion: https://postgr.es/m/9d7c5112-cb99-6a47-d3be-cf1ee6862a1d@lab.ntt.co.jp

src/include/nodes/plannodes.h

@@ -1103,6 +1103,7 @@ typedef struct PartitionPruneInfo
  * it is -1 if the partition is a leaf or has been pruned.  Note that subplan
  * indexes, as stored in 'subplan_map', are global across the parent plan
  * node, but partition indexes are valid only within a particular hierarchy.
+ * relid_map[p] contains the partition's OID, or 0 if the partition was pruned.
  */
 typedef struct PartitionedRelPruneInfo
 {
@@ -1115,7 +1116,7 @@ typedef struct PartitionedRelPruneInfo
 	int			nexprs;			/* Length of hasexecparam[] */
 	int		   *subplan_map;	/* subplan index by partition index, or -1 */
 	int		   *subpart_map;	/* subpart index by partition index, or -1 */
-	Oid		   *relid_map;		/* relation OID by partition index, or -1 */
+	Oid		   *relid_map;		/* relation OID by partition index, or 0 */
 	bool	   *hasexecparam;	/* true if corresponding pruning_step contains
 								 * any PARAM_EXEC Params. */
 	bool		do_initial_prune;	/* true if pruning should be performed

src/include/optimizer/inherit.h

@@ -17,6 +17,11 @@
 #include "nodes/pathnodes.h"
 
 
-extern void expand_inherited_tables(PlannerInfo *root);
+extern void expand_inherited_rtentry(PlannerInfo *root, RelOptInfo *rel,
+						 RangeTblEntry *rte, Index rti);
+
+extern bool apply_child_basequals(PlannerInfo *root, RelOptInfo *parentrel,
+					  RelOptInfo *childrel, RangeTblEntry *childRTE,
+					  AppendRelInfo *appinfo);
 
 #endif							/* INHERIT_H */

src/include/optimizer/pathnode.h

@@ -277,10 +277,9 @@ extern Path *reparameterize_path_by_child(PlannerInfo *root, Path *path,
  */
 extern void setup_simple_rel_arrays(PlannerInfo *root);
 extern void setup_append_rel_array(PlannerInfo *root);
+extern void expand_planner_arrays(PlannerInfo *root, int add_size);
 extern RelOptInfo *build_simple_rel(PlannerInfo *root, int relid,
 				 RelOptInfo *parent);
-extern void add_appendrel_other_rels(PlannerInfo *root, RelOptInfo *rel,
-						 Index rti);
 extern RelOptInfo *find_base_rel(PlannerInfo *root, int relid);
 extern RelOptInfo *find_join_rel(PlannerInfo *root, Relids relids);
 extern RelOptInfo *build_join_rel(PlannerInfo *root,