Add support for ALTER TABLE IF EXISTS ... RENAME CONSTRAINT

Also add regression test.  Previously this was documented to work, but
didn't.

Author: bmomjian

src/backend/commands/tablecmds.c

@@ -2497,9 +2497,16 @@ RenameConstraint(RenameStmt *stmt)
 	{
 		/* lock level taken here should match rename_constraint_internal */
 		relid = RangeVarGetRelidExtended(stmt->relation, AccessExclusiveLock,
-										 false, false,
+										 stmt->missing_ok, false,
 										 RangeVarCallbackForRenameAttribute,
 										 NULL);
+		if (!OidIsValid(relid))
+		{
+			ereport(NOTICE,
+					(errmsg("relation \"%s\" does not exist, skipping",
+							stmt->relation->relname)));
+			return InvalidObjectAddress;
+		}
 	}
 
 	return

src/backend/parser/gram.y

@@ -7706,6 +7706,17 @@ RenameStmt: ALTER AGGREGATE func_name aggr_args RENAME TO name
 					n->relation = $3;
 					n->subname = $6;
 					n->newname = $8;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			| ALTER TABLE IF_P EXISTS relation_expr RENAME CONSTRAINT name TO name
+				{
+					RenameStmt *n = makeNode(RenameStmt);
+					n->renameType = OBJECT_TABCONSTRAINT;
+					n->relation = $5;
+					n->subname = $8;
+					n->newname = $10;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			| ALTER FOREIGN TABLE relation_expr RENAME opt_column name TO name

src/test/regress/expected/alter_table.out

@@ -289,6 +289,8 @@ Inherits: constraint_rename_test
 
 DROP TABLE constraint_rename_test2;
 DROP TABLE constraint_rename_test;
+ALTER TABLE IF EXISTS constraint_not_exist RENAME CONSTRAINT con3 TO con3foo; -- ok
+NOTICE:  relation "constraint_not_exist" does not exist, skipping
 ALTER TABLE IF EXISTS constraint_rename_test ADD CONSTRAINT con4 UNIQUE (a);
 NOTICE:  relation "constraint_rename_test" does not exist, skipping
 -- FOREIGN KEY CONSTRAINT adding TEST

src/test/regress/sql/alter_table.sql

@@ -228,6 +228,7 @@ ALTER TABLE constraint_rename_test RENAME CONSTRAINT con3 TO con3foo; -- ok
 \d constraint_rename_test2
 DROP TABLE constraint_rename_test2;
 DROP TABLE constraint_rename_test;
+ALTER TABLE IF EXISTS constraint_not_exist RENAME CONSTRAINT con3 TO con3foo; -- ok
 ALTER TABLE IF EXISTS constraint_rename_test ADD CONSTRAINT con4 UNIQUE (a);
 
 -- FOREIGN KEY CONSTRAINT adding TEST