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/foreign/foreign.c

@@ -23,6 +23,8 @@
 #include "lib/stringinfo.h"
 #include "miscadmin.h"
 #include "utils/builtins.h"
+#include "utils/memutils.h"
+#include "utils/rel.h"
 #include "utils/syscache.h"
 
 
@@ -352,6 +354,50 @@ GetFdwRoutineByRelId(Oid relid)
 	return GetFdwRoutine(fdwhandler);
 }
 
+/*
+ * GetFdwRoutineForRelation - look up the handler of the foreign-data wrapper
+ * for the given foreign table, and retrieve its FdwRoutine struct.
+ *
+ * This function is preferred over GetFdwRoutineByRelId because it caches
+ * the data in the relcache entry, saving a number of catalog lookups.
+ *
+ * If makecopy is true then the returned data is freshly palloc'd in the
+ * caller's memory context.  Otherwise, it's a pointer to the relcache data,
+ * which will be lost in any relcache reset --- so don't rely on it long.
+ */
+FdwRoutine *
+GetFdwRoutineForRelation(Relation relation, bool makecopy)
+{
+	FdwRoutine *fdwroutine;
+	FdwRoutine *cfdwroutine;
+
+	if (relation->rd_fdwroutine == NULL)
+	{
+		/* Get the info by consulting the catalogs and the FDW code */
+		fdwroutine = GetFdwRoutineByRelId(RelationGetRelid(relation));
+
+		/* Save the data for later reuse in CacheMemoryContext */
+		cfdwroutine = (FdwRoutine *) MemoryContextAlloc(CacheMemoryContext,
+														sizeof(FdwRoutine));
+		memcpy(cfdwroutine, fdwroutine, sizeof(FdwRoutine));
+		relation->rd_fdwroutine = cfdwroutine;
+
+		/* Give back the locally palloc'd copy regardless of makecopy */
+		return fdwroutine;
+	}
+
+	/* We have valid cached data --- does the caller want a copy? */
+	if (makecopy)
+	{
+		fdwroutine = (FdwRoutine *) palloc(sizeof(FdwRoutine));
+		memcpy(fdwroutine, relation->rd_fdwroutine, sizeof(FdwRoutine));
+		return fdwroutine;
+	}
+
+	/* Only a short-lived reference is needed, so just hand back cached copy */
+	return relation->rd_fdwroutine;
+}
+
 
 /*
  * deflist_to_tuplestore - Helper function to convert DefElem list to