函数中执行alter语句报错

函数定义如下：
create or replace function UniMonDB.updateMenus() return int is
pragma autonomous_transaction;
begin

if not exists(SELECT NAME FROM SYSCOLUMNS WHERE ID = (SELECT ID FROM SYSOBJECTS WHERE NAME LIKE 'Tbl_aaa') AND NAME LIKE 'uiddomainid') then 
	ALTER TABLE Tbl_aaa ADD uiddomainid varchar(128);
    update Tbl_aaa set uiddomainid = (select uiddomainid from tbl_bbb limit 1) where uidDomainID = '' or uidDomainID is null;
end if;
commit;
return 1;

end;