Split out CreateCast into src/backend/catalog/pg_cast.c

This catalog-handling code was previously together with the rest of
CastCreate() in src/backend/commands/functioncmds.c.  A future patch
will need a way to add casts internally, so this will be useful to have
separate.

Also, move the nearby get_cast_oid() function from functioncmds.c to
lsyscache.c, which seems a more natural place for it.

Author: Paul Jungwirth, minor edits by Álvaro
Discussion: https://postgr.es/m/20200309210003.GA19992@alvherre.pgsql

src/backend/utils/cache/lsyscache.c

@@ -23,6 +23,7 @@
 #include "catalog/pg_am.h"
 #include "catalog/pg_amop.h"
 #include "catalog/pg_amproc.h"
+#include "catalog/pg_cast.h"
 #include "catalog/pg_collation.h"
 #include "catalog/pg_constraint.h"
 #include "catalog/pg_language.h"
@@ -908,6 +909,31 @@ get_atttypetypmodcoll(Oid relid, AttrNumber attnum,
 	ReleaseSysCache(tp);
 }
 
+/*				---------- PG_CAST CACHE ----------					 */
+
+/*
+ * get_cast_oid - given two type OIDs, look up a cast OID
+ *
+ * If missing_ok is false, throw an error if the cast is not found.  If
+ * true, just return InvalidOid.
+ */
+Oid
+get_cast_oid(Oid sourcetypeid, Oid targettypeid, bool missing_ok)
+{
+	Oid			oid;
+
+	oid = GetSysCacheOid2(CASTSOURCETARGET, Anum_pg_cast_oid,
+						  ObjectIdGetDatum(sourcetypeid),
+						  ObjectIdGetDatum(targettypeid));
+	if (!OidIsValid(oid) && !missing_ok)
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_OBJECT),
+				 errmsg("cast from type %s to type %s does not exist",
+						format_type_be(sourcetypeid),
+						format_type_be(targettypeid))));
+	return oid;
+}
+
 /*				---------- COLLATION CACHE ----------					 */
 
 /*