gaussdb建立触发器问题

gaussdb创建触发器遇到了问题，麻烦解决一下

--建表
CREATE TABLE T1
(
ID NUMBER(14,20) NOT NULL PRIMARY KEY,
WAFER_START_TIME TIMESTAMP
);

CREATE TABLE T2
(
ID NUMBER(14,20) NOT NULL PRIMARY KEY,
WAFER_START_TIME TIMESTAMP
);

-- 插入数据
INSERT T1 
VALUES 
(1001,null);

INSERT T1 
VALUES 
(11002,NOW());


-- 创建函数
CREATE OR REPLACE FUNCTION tri_update_func() 
RETURNS TRIGGER AS $TRIGGER_UPDATE$
           BEGIN
               IF OLD.birth IS NULL AND NEW.birth IS NOT NULL THEN
                  INSERT INTO T2(ID,birth)
                  VALUES (NEW.ID,NEW.birth);
               END IF;   
                   RETURN NEW;
           END;
           $TRIGGER_UPDATE$ LANGUAGE PLPGSQL;

--创建UPDATE触发器
CREATE TRIGGER TRIGGER_UPDATE
           AFTER UPDATE ON TH_Z_BACKUP_TEST1
           FOR EACH ROW
           BEGIN
           EXECUTE PROCEDURE tri_update_func();
           END;

-- 通过update语句触发触发器报错
UPDATE T1 a 
SET a.WAFER_START_TIME  = NOW()
WHERE a.ID ='1001';

[4:20] PLC-00601 Sql syntax error: IMMEDIATE expected
这个报错非常古怪根本没法执行，谁能解答一下