小编Pat*_*rry的帖子

我有 2 个组，我试图找到它们的交集（需要 2 列匹配），我发现加入 2 个临时表的性能比仅用一个临时表加入原始表的性能要慢 50 倍。这对我来说毫无意义，所以也许有人可以启发我？

这是我编写 2 个临时表版本的方式：

CREATE TEMPORARY TABLE attendees (
    event_id SMALLINT(5) UNSIGNED,
    person_id INT(10) UNSIGNED NOT NULL,
    KEY(event_id),
    KEY(person_id)
);
INSERT INTO attendees (event_id, person_id) 
    SELECT event_id, person_id
    FROM attendance WHERE year=2013
    GROUP BY event_id, person_id;
CREATE TEMPORARY TABLE invitees (
    event_id SMALLINT(5) UNSIGNED,
    person_id INT(10) UNSIGNED NOT NULL,
    KEY(event_id),
    KEY(person_id)
);
INSERT INTO invitees (event_id, person_id)
    SELECT event_id, person_id
    FROM invitations WHERE year=2013
    GROUP BY event_id, person_id;
SELECT i.event_id, COUNT(DISTINCT i.person_id) …