SELECT 和多列的 INSERT 之间的竞争条件

Question

注意：这是 this solution 的后续问题。您需要阅读 link 以获取此问题的上下文。此外，这是针对 postgres v9.4

如果我们现在想 return 多列而不是只有 1 列，我们该如何实现？

我们来 table t:

 create table t(tag_id int, tag text unique);

现在这就是我想要的：每当我调用方法 f_tag_id 时，我希望它 return 唯一行的所有列（如果它存在于 table t 中，否则插入它并 return 所有列。

所以这些是我为 f_insert_tag

尝试的东西

选项 1:

CREATE OR REPLACE FUNCTION f_insert_tag(tag_p_id int, _tag text)
RETURNS TABLE(_tag_p_id int, _tag_ text) 
AS
$func$
    BEGIN
        INSERT INTO t(tag_id,tag) VALUES (tag_p_id, _tag);
        return query Select * from t where t.tag_id = tag_p_id;
        EXCEPTION WHEN UNIQUE_VIOLATION THEN  -- catch exception, NULL is returned
    END
$func$ LANGUAGE plpgsql;

选项 2：

   CREATE OR REPLACE FUNCTION f_insert_tag(tag_p_id int, _tag text, out _tag_id int, out _tag_ text) 
AS
$func$
    BEGIN
        INSERT INTO t(tag_id,tag) VALUES (tag_p_id, _tag);
        Select t.tag_id, t.tag from t where t.tag_id = tag_p_id into _tag_id, _tag_;
        EXCEPTION WHEN UNIQUE_VIOLATION THEN  -- catch exception, NULL is returned
    END
$func$ LANGUAGE plpgsql;

选项 3：

CREATE OR REPLACE FUNCTION f_insert_tag(tag_p_id int, _tag text)
returns setof t AS
$func$
  BEGIN
    INSERT INTO t(tag_id,tag) VALUES (tag_p_id, _tag);
    return query Select * from t where t.tag_id = tag_p_id;
    EXCEPTION WHEN UNIQUE_VIOLATION THEN  -- catch exception, NULL is returned
  END
$func$ LANGUAGE plpgsql;

3个都是自己完成的：

select f_insert_tag(1322, 'helloworldaa');
    f_insert_tag
---------------------
 (1322,helloworldaa)

另外一个功能，f_tag_id我也试了很多方法：

选项 1：

CREATE OR REPLACE FUNCTION f_tag_id(tag_p_id int, _tag text, out _tag_id int, out _tag_ text) 
AS
$func$
BEGIN
   LOOP
      SELECT t.tag_id, t.tag FROM t WHERE t.tag = _tag
      UNION ALL
      SELECT f_insert_tag(tag_p_id, _tag)
      into _tag_id, _tag_;

      EXIT WHEN _tag_id IS NOT NULL;  -- else keep looping
   END LOOP;
END
$func$ LANGUAGE plpgsql;

选项 2：

CREATE OR REPLACE FUNCTION f_tag_id(tag_p_id int, _tag text)
RETURNS table(_tag_id int, _tag_ text) AS
$func$

BEGIN
   LOOP
      SELECT t.tag_id, t.tag FROM t WHERE t.tag = _tag
      UNION  ALL
      SELECT f_insert_tag(tag_p_id, _tag)
      into _tag_id, _tag_;

      EXIT WHEN _tag_id IS NOT NULL;  -- else keep looping
   END LOOP;
END
$func$ LANGUAGE plpgsql;

对于这两个，我得到了同样的错误：

select f_tag_id(22, 'test');
ERROR:  each UNION query must have the same number of columns
LINE 3:       SELECT f_insert_tag(tag_p_id, _tag)
                     ^
QUERY:  SELECT t.tag_id, t.tag FROM t WHERE t.tag = _tag
      UNION  ALL
      SELECT f_insert_tag(tag_p_id, _tag)
CONTEXT:  PL/pgSQL function f_tag_id(integer,text) line 5 at SQL statement

Answer 1

作品中的扳手是 ~~SELECT f_insert_tag(tag_p_id, _tag)~~ 而不是

SELECT * FROM f_insert_tag(tag_p_id, _tag)

对于 Postgres 9.4

CREATE FUNCTION f_insert_tag(_tag_id int, _tag text, OUT _tag_id_ int, OUT _tag_ text) 
AS
$func$
 BEGIN
   INSERT INTO t(tag_id, tag)
   VALUES (_tag_id, _tag)
   RETURNING t.tag_id, t.tag
   INTO  _tag_id_, _tag_;

   EXCEPTION WHEN UNIQUE_VIOLATION THEN
      -- catch exception, return NULL
 END
$func$  LANGUAGE plpgsql;


CREATE FUNCTION f_tag_id(_tag_id int, _tag text, OUT _tag_id_ int, OUT _tag_ text) AS
$func$
BEGIN
LOOP
   SELECT t.tag_id, t.tag
   FROM   t
   WHERE  t.tag = _tag

   UNION ALL
   SELECT *                                              -- !!!
   FROM   f_insert_tag(_tag_id, _tag)
   LIMIT  1

   INTO _tag_id_, _tag_;

   EXIT WHEN _tag_id_ IS NOT NULL;  -- else keep looping
END LOOP;
END
$func$ LANGUAGE plpgsql;

db<>fiddle here

对于 Postgres 9.5 或更高版本：

CREATE FUNCTION f_tag_id(_tag_id int, _tag text, OUT _tag_id_ int, OUT _tag_ text) AS
$func$
BEGIN
LOOP
   SELECT t.tag_id, t.tag
   FROM   t
   WHERE  t.tag = _tag
   INTO   _tag_id_, _tag_;

   EXIT WHEN FOUND;

   INSERT INTO t (tag_id, tag)
   VALUES (_tag_id, _tag)
   ON     CONFLICT (tag) DO NOTHING
   RETURNING t.tag_id, t.tag
   INTO   _tag_id_, _tag_;

   EXIT WHEN FOUND;
END LOOP;
END
$func$  LANGUAGE plpgsql;

db<>fiddle here

这里是基础知识：

Is SELECT or INSERT in a function prone to race conditions?

SELECT 和多列的 INSERT 之间的竞争条件

Race Condition between SELECT and INSERT for multiple columns

sql

postgresql

concurrency

upsert

plpgsql

对于 Postgres 9.4

对于 Postgres 9.5 或更高版本：