postgresql函数变形

以下三个函数等价

CREATE OR REPLACE FUNCTION func_mppd30101() RETURNS INTEGER AS
$BODY$
BEGIN
    perform * FROM t WHERE id = 3;
    if not found then
        INSERT INTO t VALUES (3);
    end if;
    return 0;
END;
$BODY$
LANGUAGE 'plpgsql' ;

如果改成select，则报以下错误
ERROR:  query has no destination for result data 
HINT:  If you want to discard the results of a SELECT, use PERFORM instead.

CREATE OR REPLACE FUNCTION func_mppd30101() RETURNS INTEGER AS
$BODY$
declare
a int;
BEGIN
    select * FROM t WHERE id = 3 into a;
    if not found then
        INSERT INTO t VALUES (3);
    end if;
    return 0;
END;
$BODY$
LANGUAGE 'plpgsql' ;

CREATE OR REPLACE FUNCTION func_mppd30101() RETURNS INTEGER AS
$BODY$
BEGIN
    if (not exists (select * FROM t WHERE id = 4)) then
        INSERT INTO t VALUES (4);
    end if;
    return 0;
END;
$BODY$
LANGUAGE 'plpgsql' ;