Arrange to cache FdwRoutine structs in foreign tables' relcache entries.

This saves several catalog lookups per reference.  It's not all that
exciting right now, because we'd managed to minimize the number of places
that need to fetch the data; but the upcoming writable-foreign-tables patch
needs this info in a lot more places.

src/backend/optimizer/path/allpaths.c

@@ -410,9 +410,6 @@ set_foreign_size(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 	/* Mark rel with estimated output rows, width, etc */
 	set_foreign_size_estimates(root, rel);
 
-	/* Get FDW routine pointers for the rel */
-	rel->fdwroutine = GetFdwRoutineByRelId(rte->relid);
-
 	/* Let FDW adjust the size estimates, if it can */
 	rel->fdwroutine->GetForeignRelSize(root, rel, rte->relid);
 }

src/backend/optimizer/util/plancat.c

@@ -26,6 +26,7 @@
 #include "access/xlog.h"
 #include "catalog/catalog.h"
 #include "catalog/heap.h"
+#include "foreign/fdwapi.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
 #include "optimizer/clauses.h"
@@ -67,6 +68,7 @@ static List *build_index_tlist(PlannerInfo *root, IndexOptInfo *index,
  *	min_attr	lowest valid AttrNumber
  *	max_attr	highest valid AttrNumber
  *	indexlist	list of IndexOptInfos for relation's indexes
+ *	fdwroutine	if it's a foreign table, the FDW function pointers
  *	pages		number of pages
  *	tuples		number of tuples
  *
@@ -374,6 +376,12 @@ get_relation_info(PlannerInfo *root, Oid relationObjectId, bool inhparent,
 
 	rel->indexlist = indexinfos;
 
+	/* Grab the fdwroutine info using the relcache, while we have it */
+	if (relation->rd_rel->relkind == RELKIND_FOREIGN_TABLE)
+		rel->fdwroutine = GetFdwRoutineForRelation(relation, true);
+	else
+		rel->fdwroutine = NULL;
+
 	heap_close(relation, NoLock);
 
 	/*