Question

我构建了以下查询：

SELECT
    p.id,
    p.tag_id,
    p.title,
    p.created_at,
    @indexer := @indexer + 1 AS indexer
FROM 
    `posts` AS p
LEFT JOIN
    `votes` AS v
        ON p.id = v.votable_id
        AND v.votable_type = "Post"
        AND v.deleted_at IS NULL
JOIN
    (SELECT @indexer := 0) AS i
WHERE
    p.deleted_at IS NULL
GROUP BY
    p.id

我得到的结果是：

+----+--------+------------------------------------+---------------------+---------+
| id | tag_id |               title                |     created_at      | indexer |
+----+--------+------------------------------------+---------------------+---------+
|  2 |      2 | PostPostPost                       | 2014-10-23 23:53:15 |     248 |
|  3 |      3 | Title                              | 2014-10-23 23:56:13 |       6 |
|  4 |      2 | GIFGIFIGIIF                        | 2014-10-23 23:59:03 |    1316 |
|  5 |      2 | GIFGIFIGIIF                        | 2014-10-23 23:59:03 |    1317 |
|  6 |      4 | My new avatar                      | 2014-10-26 22:22:30 |    1318 |
|  7 |      5 | Hi, haiii, oh Hey !                | 2014-10-26 22:38:10 |       1 |
|  8 |      6 | Mclaren testing stealth technology | 2014-10-26 22:44:15 |       5 |
|  9 |      7 | Just random thoughts while pooping | 2014-10-26 22:50:03 |       2 |
+----+--------+------------------------------------+---------------------+---------+

然而，我期待着这一点：

+----+--------+------------------------------------+---------------------+---------+
| id | tag_id |               title                |     created_at      | indexer |
+----+--------+------------------------------------+---------------------+---------+
|  2 |      2 | PostPostPost                       | 2014-10-23 23:53:15 |       1 |
|  3 |      3 | Title                              | 2014-10-23 23:56:13 |       2 |
|  4 |      2 | GIFGIFIGIIF                        | 2014-10-23 23:59:03 |       3 |
|  5 |      2 | GIFGIFIGIIF                        | 2014-10-23 23:59:03 |       4 |
|  6 |      4 | My new avatar                      | 2014-10-26 22:22:30 |       5 |
|  7 |      5 | Hi, haiii, oh Hey !                | 2014-10-26 22:38:10 |       6 |
|  8 |      6 | Mclaren testing stealth technology | 2014-10-26 22:44:15 |       7 |
|  9 |      7 | Just random thoughts while pooping | 2014-10-26 22:50:03 |       8 |
+----+--------+------------------------------------+---------------------+---------+

提示：问题在于indexer字段。它是在添加GROUP BY语句后发生的。

我尝试移动JOIN (SELECT @indexer := 0) AS i，将其更改为额外的选择，但没有更改。无论如何，指数都搞砸了。

如何解决这个问题，以便索引正确？
这个问题背后的原因是什么？

Answer 1

我自己找到答案，不得不将查询分成多个子查询。

SELECT
    mq.*,
    @indexer := @indexer + 1 AS indexer
FROM
(
    SELECT
        p.id,
        p.tag_id,
        p.title,
        p.created_at
    FROM 
        `posts` AS p
    LEFT JOIN
        `votes` AS v
            ON p.id = v.votable_id
            AND v.votable_type = "Post"
            AND v.deleted_at IS NULL
    WHERE
        p.deleted_at IS NULL
    GROUP BY
        p.id
) AS mq
JOIN
    (SELECT @indexer := 0) AS i

显然，这导致查询的大量访问行（按照EXPLAIN），但设法通过额外的索引来修复它。可以在此处找到该问题的完整答案：Calculating row indices with subquery having joins, results in A*B examined rows

分组时行索引不正确

1 个答案: