多次运行不会报错

DO language plpgsql $$ BEGIN RAISE INFO '*************** test_add_column.sql begins'' ***************';
    if not exists (select 1 from information_schema.columns where table_schema = 'public' and table_name = 'qer_simutrade_param' and column_name = 'pool_id') then RAISE INFO '*************** column npool_id not exist,add now... ***************';
        alter table public.qer_simutrade_param add column pool_id varchar;
    end if;
    RAISE INFO '*************** test_add_column.sql completes'' ***************';
    EXCEPTION
      WHEN OTHERS THEN
        RAISE NOTICE '*************** Patch ''test_add_column.sql'' fails : ErrCode (%) (%)',sqlSTATE,sqlERRM;
        RAISE;
END;
$$;