Greenplum中exit,return和label的注意事项

Greenplum中exit和return的注意事项


1. 先来看exit

CREATE OR REPLACE FUNCTION test_exit()
RETURNS integer AS
$$
DECLARE 
    count int;
BEGIN
    count := 1;
    LOOP
        count := count + 1;


        begin         
            raise notice '<<<begin start>>> xact: %', count;
            
            EXECUTE 'select pg_sleep(1)';
            IF count > 5 THEN
                EXIT;       --这里使用EXIT只会跳出了begin, 而不会跳出LOOP循环
            END IF;
            raise notice '<<<begin end>>> not exit xact: %', count;
        end;


        raise notice '<<<loop>>> xact: %', count;
        raise notice '==================================';
    END LOOP;
    return 1;
END
$$ LANGUAGE plpgsql;


zhangyun_db=> select test_exit();
NOTICE:  <<<begin start>>> xact: 2
NOTICE:  <<<begin end>>> not exit xact: 2
NOTICE:  <<<loop>>> xact: 2
NOTICE:  ==================================
NOTICE:  <<<begin start>>> xact: 3
NOTICE:  <<<begin end>>> not exit xact: 3
NOTICE:  <<<loop>>> xact: 3
NOTICE:  ==================================
NOTICE:  <<<begin start>>> xact: 4
NOTICE:  <<<begin end>>> not exit xact: 4
NOTICE:  <<<loop>>> xact: 4
NOTICE:  ==================================
NOTICE:  <<<begin start>>> xact: 5
NOTICE:  <<<begin end>>> not exit xact: 5
NOTICE:  <<<loop>>> xact: 5
NOTICE:  ==================================
NOTICE:  <<<begin start>>> xact: 6
NOTICE:  <<<loop>>> xact: 6
NOTICE:  ==================================
NOTICE:  <<<begin start>>> xact: 7
NOTICE:  <<<loop>>> xact: 7
NOTICE:  ==================================
................... 一直执行，此处省略  

可以看到当count为6时，直接跳出begin...end语句块，begin...end剩下的语句都不执行，但是并没有跳出loop循环。

2. 下面我们来看一下return语句

CREATE OR REPLACE FUNCTION test_return()
RETURNS integer AS
$$
DECLARE 
    count int;
BEGIN
    count := 1;
    LOOP
        count := count + 1;


        begin         
            raise notice '<<<begin start>>> xact: %', count;
            
            EXECUTE 'select pg_sleep(1)';
            IF count > 5 THEN
                return 0;
            END IF;
            raise notice '<<<begin end>>> not exit xact: %', count;
        end;


        raise notice '<<<loop>>> xact: %', count;
        raise notice '==================================';
    END LOOP;
    return 1;
END
$$ LANGUAGE plpgsql;

我们可以看到，改成return后，当count为6时，退出整个函数。


3. 通过使用label控制循环

CREATE OR REPLACE FUNCTION test_label()
RETURNS integer AS
$$
DECLARE 
    count int;
BEGIN
    count := 1;
    <<loop1>>
    LOOP
        count := count + 1;


        <<label>>
        begin
            EXECUTE 'select pg_sleep(1)';
            
            raise notice '<<<begin start>>> xact: %', count;
            exit loop1 when count > 5 ;


            raise notice '<<<begin end>>> not exit xact: %', count;
        end label;


        raise notice '<<<loop>>> xact: %', count;
        raise notice '==================================';
    END LOOP loop1;
    return 1;
END
$$ LANGUAGE plpgsql;<pre name="code" class="sql">zhangyun_db=> select test_return();
NOTICE:  <<<begin start>>> xact: 2
NOTICE:  <<<begin end>>> not exit xact: 2
NOTICE:  <<<loop>>> xact: 2
NOTICE:  ==================================
NOTICE:  <<<begin start>>> xact: 3
NOTICE:  <<<begin end>>> not exit xact: 3
NOTICE:  <<<loop>>> xact: 3
NOTICE:  ==================================
NOTICE:  <<<begin start>>> xact: 4
NOTICE:  <<<begin end>>> not exit xact: 4
NOTICE:  <<<loop>>> xact: 4
NOTICE:  ==================================
NOTICE:  <<<begin start>>> xact: 5
NOTICE:  <<<begin end>>> not exit xact: 5
NOTICE:  <<<loop>>> xact: 5
NOTICE:  ==================================
NOTICE:  <<<begin start>>> xact: 6
 test_return 
-------------
           0
(1 row)


zhangyun_db=> 

可以看出使用label可以灵活控制循环的阶段，当初count为6时，直接跳转到loop循环外。