CONSTRAINT

alter table tab add unique (tid,rid,cid);

select * from information_schema.constraint_column_usage;

-- 只支持 IF EXISTS
alter table tab drop constraint if exists tab_tid_rid_cid_key ;

-- try cache
DO $$
BEGIN

  BEGIN
    ALTER TABLE foo ADD CONSTRAINT bar ;
  EXCEPTION
    WHEN duplicate_table THEN  -- UNIQUE - 9.6.8
    WHEN duplicate_object THEN -- FOREIGN KEY
    WHEN invalid_table_definition THEN -- PRIMARY KEY - 11.9
      RAISE NOTICE 'Table constraint foo.bar already exists';
  END;

END $$;