MySQL花费更多时间发送数据

提问

我有一个特殊的查询,它需要很长时间才能执行,同一表上的其他查询执行得非常快. mysql中启用了Querycache,但是每次下面的查询每次都花费超过80秒,并且CPU利用率超过100%.

我无法修改查询,因为它是由Drupal生成的.我还有什么可以做以提高性能?

查询是:

select count(*) 
from (
    SELECT slk.key_id AS key_id 
    FROM slk slk  
        LEFT JOIN users users ON    slk.uid = users.uid 
        LEFT JOIN node node_users ON users.uid = node_users.uid 
            AND   node_users.type = 'profile'
) count_alias;

以下是个人资料信息:

+--------------------------------+-----------+
| Status                         | Duration  |
+--------------------------------+-----------+
| starting                       |  0.000029 | 
| checking query cache for query |  0.000093 | 
| Opening tables                 |  0.000210 | 
| System lock                    |  0.000007 | 
| Table lock                     |  0.000075 | 
| optimizing                     |  0.000008 | 
| statistics                     |  0.000113 | 
| preparing                      |  0.000027 | 
| executing                      |  0.000004 | 
| Sending data                   | 66.086903 | 
| init                           |  0.000027 | 
| optimizing                     |  0.000009 | 
| executing                      |  0.000018 | 
| end                            |  0.000003 | 
| query end                      |  0.000004 | 
| freeing items                  |  0.000049 | 
| storing result in query cache  |  0.000116 | 
| removing tmp table             |  0.033162 | 
| closing tables                 |  0.000106 | 
| logging slow query             |  0.000003 | 
| logging slow query             |  0.000085 | 
| cleaning up                    |  0.000007 | 
+--------------------------------+-----------+

对查询的解释给出:

| id | select_type | table      | type   | possible_keys         | key     | key_len | ref             | rows  | Extra                        |
|  1 | PRIMARY     | NULL       | NULL   | NULL                  | NULL    | NULL    | NULL            |  NULL | Select tables optimized away | 
|  2 | DERIVED     | slk        | ALL    | NULL                  | NULL    | NULL    | NULL            | 55862 |                              | 
|  2 | DERIVED     | users      | eq_ref | PRIMARY               | PRIMARY | 4       | gscom.slk.uid   |     1 | Using index                  | 
|  2 | DERIVED     | node_users | ref    | node_type,uid,idx_ctp | uid     | 4       | gscom.users.uid |     3 |                              | 

idx_ctp是(uid,类型)上的索引.

查询缓存正在运行,下面是统计信息.

显示变量,例如’%query_cache%’;:

| Variable_name                | Value    |
| have_query_cache             | YES      | 
| query_cache_limit            | 2097152  | 
| query_cache_min_res_unit     | 4096     | 
| query_cache_size             | 52428800 | 
| query_cache_type             | ON       | 
| query_cache_wlock_invalidate | OFF      |

mysql>显示状态,例如“%Qcache%” ;:

| Variable_name           | Value    |
| Qcache_free_blocks      | 1255     | 
| Qcache_free_memory      | 22902848 | 
| Qcache_hits             | 1484908  | 
| Qcache_inserts          | 1036344  | 
| Qcache_lowmem_prunes    | 95086    | 
| Qcache_not_cached       | 3975     | 
| Qcache_queries_in_cache | 14271    | 
| Qcache_total_blocks     | 30117    | 

最佳答案

您需要以下索引:

>表slk:(uid)
>表node_users :(类型,uid)

无需子查询即可将查询重写为:

SELECT COUNT(*) 
FROM slk 
    LEFT JOIN users 
        ON slk.uid = users.uid 
    LEFT JOIN node node_users 
        ON  users.uid = node_users.uid 
        AND node_users.type = 'profile'

我真的不确定您为什么使用LEFT JOIN.您可能可以使用INNER JOIN并获得相同的结果.或只是简单地使用:

SELECT COUNT(*) 
FROM slk 
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