我有一个包含数千行内容（约300万）的Joomla表。在查询表时，我在编写数据库查询时遇到了一些麻烦。
以下是我的完整查询：
SELECT cc.title AS category, a.id, a.title, a.alias, a.title_alias, a.introtext, a.fulltext, a.sectionid, a.state, a.catid, a.created, a.created_by, a.created_by_alias, a.modified, a.modified_by, a.checked_out, a.checked_out_time, a.publish_up, a.publish_down, a.attribs, a.hits, a.images, a.urls, a.ordering, a.metakey, a.metadesc, a.access, CASE WHEN CHAR_LENGTH(a.alias) THEN CONCAT_WS(":", a.id, a.alias) ELSE a.id END AS slug, CASE WHEN CHAR_LENGTH(cc.alias) THEN CONCAT_WS(":", cc.id, cc.alias) ELSE cc.id END AS catslug, CHAR_LENGTH( a.`fulltext` ) AS readmore, u.name AS author, u.usertype, g.name AS groups, u.email AS author_email
FROM j15_content AS a
LEFT JOIN j15_categories AS cc
ON a.catid = cc.id
LEFT JOIN j15_users AS u
ON u.id = a.created_by
LEFT JOIN j15_groups AS g
ON a.access = g.id
WHERE 1
AND a.access <= 0
AND a.catid = 108
AND a.state = 1
AND ( publish_up = '0000-00-00 00:00:00' OR publish_up <= '2012-02-08 00:16:26' )
AND ( publish_down = '0000-00-00 00:00:00' OR publish_down >= '2012-02-08 00:16:26' )
ORDER BY a.title, a.created DESC
LIMIT 0, 10
以下是EXPLAIN的输出：
+----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-----------------------------+
| 1 | SIMPLE | a | ref | idx_access,idx_state,idx_catid,idx_access_state_catid | idx_catid | 4 | const | 3108187 | Using where; Using filesort |
| 1 | SIMPLE | cc | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| 1 | SIMPLE | u | eq_ref | PRIMARY | PRIMARY | 4 | database.a.created_by | 1 | |
| 1 | SIMPLE | g | eq_ref | PRIMARY | PRIMARY | 1 | database.a.access | 1 | |
+----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-----------------------------+
为了显示存在哪些索引，SHOW INDEX FROM j15_content：
+-------------+------------+------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------------+------------+------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| j15_content | 0 | PRIMARY | 1 | id | A | 3228356 | NULL | NULL | | BTREE | |
| j15_content | 1 | idx_section | 1 | sectionid | A | 2 | NULL | NULL | | BTREE | |
| j15_content | 1 | idx_access | 1 | access | A | 1 | NULL | NULL | | BTREE | |
| j15_content | 1 | idx_checkout | 1 | checked_out | A | 2 | NULL | NULL | | BTREE | |
| j15_content | 1 | idx_state | 1 | state | A | 2 | NULL | NULL | | BTREE | |
| j15_content | 1 | idx_catid | 1 | catid | A | 6 | NULL | NULL | | BTREE | |
| j15_content | 1 | idx_createdby | 1 | created_by | A | 1 | NULL | NULL | | BTREE | |
| j15_content | 1 | title | 1 | title | A | 201772 | 4 | NULL | | BTREE | |
| j15_content | 1 | idx_access_state_catid | 1 | access | A | 1 | NULL | NULL | | BTREE | |
| j15_content | 1 | idx_access_state_catid | 2 | state | A | 2 | NULL | NULL | | BTREE | |
| j15_content | 1 | idx_access_state_catid | 3 | catid | A | 7 | NULL | NULL | | BTREE | |
| j15_content | 1 | idx_title_created | 1 | title | A | 3228356 | 8 | NULL | | BTREE | |
| j15_content | 1 | idx_title_created | 2 | created | A | 3228356 | NULL | NULL | | BTREE | |
+-------------+------------+------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
正如您所看到的，从数据库中获取了一些数据。现在我通过简化查询来测试真正的问题在于ORDER BY子句。如果没有对结果进行排序，查询就会非常敏感，这里有一个解释：
+----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-------------+
| 1 | SIMPLE | a | ref | idx_access,idx_state,idx_catid,idx_access_state_catid | idx_catid | 4 | const | 3108187 | Using where |
| 1 | SIMPLE | cc | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| 1 | SIMPLE | u | eq_ref | PRIMARY | PRIMARY | 4 | database.a.created_by | 1 | |
| 1 | SIMPLE | g | eq_ref | PRIMARY | PRIMARY | 1 | database.a.access | 1 | |
+----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-------------+
正如你所看到的那样，正在杀死服务器的致命文件夹。有了这么多行，我正在尽力通过索引来优化所有内容，但有些东西仍然是不对的。任何意见都将不胜感激。
尝试使用FORCE INDEX无济于事：
explain SELECT cc.title AS category, a.id, a.title, a.alias, a.title_alias, a.introtext, a.fulltext, a.sectionid, a.state, a.catid, a.created, a.created_by, a.created_by_alias, a.modified, a.modified_by, a.checked_out, a.checked_out_time, a.publish_up, a.publish_down, a.attribs, a.hits, a.images, a.urls, a.ordering, a.metakey, a.metadesc, a.access, CASE WHEN CHAR_LENGTH(a.alias) THEN CONCAT_WS(":", a.id, a.alias) ELSE a.id END AS slug, CASE WHEN CHAR_LENGTH(cc.alias) THEN CONCAT_WS(":", cc.id, cc.alias) ELSE cc.id END AS catslug, CHAR_LENGTH( a.`fulltext` ) AS readmore, u.name AS author, u.usertype, g.name AS groups, u.email AS author_email
-> FROM bak_content AS a
-> FORCE INDEX (idx_title_created)
-> LEFT JOIN bak_categories AS cc
-> ON a.catid = cc.id
-> LEFT JOIN bak_users AS u
-> ON u.id = a.created_by
-> LEFT JOIN bak_groups AS g
-> ON a.access = g.id
-> WHERE 1
-> AND a.access <= 0
-> AND a.catid = 108
-> AND a.state = 1
-> AND ( publish_up = '0000-00-00 00:00:00' OR publish_up <= '2012-02-08
-> AND ( publish_down = '0000-00-00 00:00:00' OR publish_down >= '2012-0
-> ORDER BY a.title, a.created DESC
-> LIMIT 0, 10;
产地：
+----+-------------+-------+--------+---------------+---------+---------+-------
| id | select_type | table | type | possible_keys | key | key_len | ref
+----+-------------+-------+--------+---------------+---------+---------+-------
| 1 | SIMPLE | a | ALL | NULL | NULL | NULL | NULL
| 1 | SIMPLE | cc | const | PRIMARY | PRIMARY | 4 | const
| 1 | SIMPLE | u | eq_ref | PRIMARY | PRIMARY | 4 | database
| 1 | SIMPLE | g | eq_ref | PRIMARY | PRIMARY | 1 | database
+----+-------------+-------+--------+---------------+---------+---------+-------
答案 0 :(得分：5)
AFAIK使用索引，提示或重构查询本身无法合理解决这个问题。
这个问题很慢的原因是它需要一个2M行的文件排，这实际上需要很长时间。如果您按照指定ORDER BY a.title, a.created DESC放大订单。问题是在多于1列上进行排序和具有DESC部分的组合。 Mysql不支持降序索引（CREATE INDEX statement支持关键字DESC，但仅供将来使用）。
建议的解决方法是创建一个额外的列'reverse_created'，它会自动填充，以便您的查询可以使用ORDER BY a.title, a.reverse_created。所以你用max_time - created_time填充它。然后在该组合上创建一个索引，并（如果需要）将该索引指定为提示。
有一些关于这个主题的非常好的博客文章可以用一些例子来解释这个问题：
-Update-您应该能够通过在查询中删除订单中的“DESC”部分来对此进行快速测试。结果将在功能上出错，但它应该使用您拥有的现有索引（否则强制应该起作用）。
答案 1 :(得分：0)
有时MySQL无法找到合适的索引。您可以通过提示正确的索引来解决此问题。
提示语法： http://dev.mysql.com/doc/refman/4.1/en/index-hints.html
确保您拥有正确的索引并通过试验调整其性能。
干杯！
答案 2 :(得分：0)
你可以尝试这种变化：
SELECT cc.title AS category, ...
FROM
( SELECT *
FROM j15_content AS a
USE INDEX (title) --- with and without the hint
WHERE 1
AND a.access <= 0
AND a.catid = 108
AND a.state = 1
AND ( publish_up = '0000-00-00 00:00:00'
OR publish_up <= '2012-02-08 00:16:26' )
AND ( publish_down = '0000-00-00 00:00:00'
OR publish_down >= '2012-02-08 00:16:26' )
ORDER BY a.title, a.created DESC
LIMIT 0, 10
) AS a
LEFT JOIN j15_categories AS cc
ON a.catid = cc.id
LEFT JOIN j15_users AS u
ON u.id = a.created_by
LEFT JOIN j15_groups AS g
ON a.access = g.id
我认为(catid, state, title)的索引会更好。
答案 3 :(得分：0)
也许尝试这可能会有所帮助：
CREATE INDEX idx_catid_title_created ON j15_content (catid,title(8),created);
DROP INDEX idx_catid ON j15_content;
答案 4 :(得分：0)
答案 5 :(得分：0)
我希望这在语法上是正确的
SELECT
cc.title AS category,
a.id, a.title, a.alias, a.title_alias,
a.introtext, a.fulltext, a.sectionid,
a.state, a.catid, a.created, a.created_by,
a.created_by_alias, a.modified, a.modified_by,
a.checked_out, a.checked_out_time,
a.publish_up, a.publish_down, a.attribs,
a.hits, a.images, a.urls, a.ordering, a.metakey,
a.metadesc, a.access,
CASE WHEN CHAR_LENGTH(a.alias) THEN CONCAT_WS(":", a.id, a.alias) ELSE a.id END AS slug,
CASE WHEN CHAR_LENGTH(cc.alias) THEN CONCAT_WS(":", cc.id, cc.alias) ELSE cc.id END AS catslug, CHAR_LENGTH( a.`fulltext` ) AS readmore,
u.name AS author, u.usertype, g.name AS groups, u.email AS author_email
FROM
(
SELECT aa.*
FROM
(
SELECT id FROM
FROM j15_content
WHERE catid=108 AND state=1
AND a.access <= 0
AND (publish_up = '0000-00-00 00:00:00' OR publish_up <= '2012-02-08 00:16:26')
AND (publish_down = '0000-00-00 00:00:00' OR publish_down >= '2012-02-08 00:16:26')
ORDER BY title,created DESC
LIMIT 0,10
) needed_keys
LEFT JOIN j15_content aa USING (id)
) a
LEFT JOIN j15_categories AS cc ON a.catid = cc.id
LEFT JOIN j15_users AS u ON a.created_by = u.id
LEFT JOIN j15_groups AS g ON a.access = g.id;
您需要子查询的支持索引needed_keys
ALTER TABLE j15_content ADD INDEX subquery_ndx (catid,state,access,title,created);
试一试!!!