Allow SET TABLESPACE to database default

We've always allowed CREATE TABLE to create tables in the database's default
tablespace without checking for CREATE permissions on that tablespace.
Unfortunately, the original implementation of ALTER TABLE ... SET TABLESPACE
didn't pick up on that exception.

This changes ALTER TABLE ... SET TABLESPACE to allow the database's default
tablespace without checking for CREATE rights on that tablespace, just as
CREATE TABLE works today.  Users could always do this through a series of
commands (CREATE TABLE ... AS SELECT * FROM ...; DROP TABLE ...; etc), so
let's fix the oversight in SET TABLESPACE's original implementation.

src/backend/commands/tablecmds.c

@@ -6557,7 +6557,6 @@ static void
 ATPrepSetTableSpace(AlteredTableInfo *tab, Relation rel, char *tablespacename)
 {
 	Oid			tablespaceId;
-	AclResult	aclresult;
 
 	/* Check that the tablespace exists */
 	tablespaceId = get_tablespace_oid(tablespacename);
@@ -6566,16 +6565,22 @@ ATPrepSetTableSpace(AlteredTableInfo *tab, Relation rel, char *tablespacename)
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 				 errmsg("tablespace \"%s\" does not exist", tablespacename)));
 
-	/* Check its permissions */
-	aclresult = pg_tablespace_aclcheck(tablespaceId, GetUserId(), ACL_CREATE);
-	if (aclresult != ACLCHECK_OK)
-		aclcheck_error(aclresult, ACL_KIND_TABLESPACE, tablespacename);
+	/* Check permissions except when moving to database's default */
+	if (OidIsValid(tablespaceId) && tablespaceId != MyDatabaseTableSpace)
+	{
+		AclResult	aclresult;
+
+		aclresult = pg_tablespace_aclcheck(tablespaceId, GetUserId(), ACL_CREATE);
+		if (aclresult != ACLCHECK_OK)
+			aclcheck_error(aclresult, ACL_KIND_TABLESPACE, tablespacename);
+	}
 
 	/* Save info for Phase 3 to do the real work */
 	if (OidIsValid(tab->newTableSpace))
 		ereport(ERROR,
 				(errcode(ERRCODE_SYNTAX_ERROR),
 				 errmsg("cannot have multiple SET TABLESPACE subcommands")));
+
 	tab->newTableSpace = tablespaceId;
 }