Question

我试图仅限制父行。我有那个查询

    SELECT
  comment.id user_id,
  comment.comment,
 comment.reply_id reply_id,
    comment.user_id,
u.image image,
u.group gg2,
comment.edited,
comment.deleted,
u.id idd2,
g.verified verified,
comment.date,
u.login,
comment.news_id
FROM (SELECT c.*
      FROM comments AS c
      WHERE c.news_id = 16
      ORDER BY c.id DESC
      LIMIT 0, 15) AS base
LEFT JOIN comments AS comment ON (
                    comment.reply_id = base.id
                    OR comment.id = base.id
                )
LEFT JOIN users u ON comment.user_id = u.id
LEFT JOIN groups g ON u.group = g.id
ORDER BY comment.id DESC

我想只限制父母而不是孩子。

此查询有效但不是最佳。我有400万行，这个查询需要105.1980秒。

我在reply_id，id，user_id和news_id上有索引。你可以帮助我，优化它吗？

示例LIMIT 4 parent（reply_id）：

+-------------+----------------------+----------+
| id          | comment              | reply_id |
+-------------+----------------------+----------+
|           1 | Sample Text          |     0    |
|           2 | Sample Text          |     0    |
|           3 | Sample Text          |     0    |
|           4 | Sample Text          |     0    |
|           5 | Sample Text          |     1    |
|           6 | Sample Text          |     1    |
|           7 | Sample Text          |     4    |
|           8 | Sample Text          |     2    |
|           9 | Sample Text          |     3    |
|          10 | Sample Text          |     3    |
+-------------+----------------------+----------+

Answer 1

限制这一点不会让它变得更快：在comment_id上的顺序是（一）你的性能限制因素。

添加限制：

SELECT
comment.id user_id,
comment.comment,
comment.reply_id reply_id,
comment.user_id,
u.image image,
u.group gg2,
comment.edited,
comment.deleted,
u.id idd2,
g.verified verified,
comment.date,
u.login,
comment.news_id
FROM (SELECT c.*
  FROM comments AS c
  WHERE c.news_id = 16
  ORDER BY c.id DESC
  LIMIT 0, 15) AS base
LEFT JOIN comments AS comment FORCE INDEX (idx_comment_nn_1) ON 
                comment.reply_id = base.id
                OR comment.id = base.id
LEFT JOIN users u ON comment.user_id = u.id
LEFT JOIN groups g ON u.group = g.id
ORDER BY comment.id DESC
LIMIT 1;

添加此索引可能会更快一点：

CREATE INDEX idx_comment_nn_1 ON comments(id DESC);

解释计划必须告诉你它是否被使用。

试图限制父行

1 个答案: