SQL_NO_CACHE不起作用

Question

我第一次运行这个sql,需要39秒,当我再次运行并增加SQL_NO_CACHE时,似乎没有生效:

mysql> select count(*) from `deal_expired` where `site`=8&&`area`=122 && 
endtime<1310444996056;
+----------+
| count(*) |
+----------+
|      497 |
+----------+
1 row in set (39.55 sec)

mysql> select SQL_NO_CACHE count(*) from `deal_expired` where `site`=8&&`area`=
122 && endtime<1310444996056;
+----------+
| count(*) |
+----------+
|      497 |
+----------+
1 row in set (0.16 sec)

我在这里尝试了各种方法

甚至重启mysql服务器或更改表名,但我还是不能让39秒运行这个SQL

我替换了另一个SQL,并在SQL_NO_CACHE上第一次运行增加,问题是一样的:

mysql> select SQL_NO_CACHE count(*) from `deal_expired` where `site`=25&&`area`=
134 && endtime<1310483196227;
+----------+
| count(*) |
+----------+
|      315 |
+----------+
1 row in set (2.17 sec)

mysql> select SQL_NO_CACHE count(*) from `deal_expired` where `site`=25&&`area`=
134 && endtime<1310483196227;
+----------+
| count(*) |
+----------+
|      315 |
+----------+
1 row in set (0.01 sec)

是什么原因？如何获得相同的SQL运行时？

我想找到一种方法来优化这个SQL以执行39秒

顺便说一句:RESET QUERY CACHE FLUSH QUERY CACHE FLUSH TABLES SET SESSION query_cache_type=off不行

mysql状态缓存已关闭:

mysql> SHOW STATUS LIKE "Qcache%";
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Qcache_free_blocks      | 0     |
| Qcache_free_memory      | 0     |
| Qcache_hits             | 0     |
| Qcache_inserts          | 0     |
| Qcache_lowmem_prunes    | 0     |
| Qcache_not_cached       | 0     |
| Qcache_queries_in_cache | 0     |
| Qcache_total_blocks     | 0     |
+-------------------------+-------+
8 rows in set (0.04 sec)

mysql> select count(*) from `deal_expired` where `site`=25&&`area`=134 && endtime<1310
483196227;
+----------+
| count(*) |
+----------+
|      315 |
+----------+
1 row in set (0.01 sec)

mysql> SHOW STATUS LIKE "Qcache%";
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Qcache_free_blocks      | 0     |
| Qcache_free_memory      | 0     |
| Qcache_hits             | 0     |
| Qcache_inserts          | 0     |
| Qcache_lowmem_prunes    | 0     |
| Qcache_not_cached       | 0     |
| Qcache_queries_in_cache | 0     |
| Qcache_total_blocks     | 0     |
+-------------------------+-------+
8 rows in set (0.00 sec)

---

解释这个SQL,使用site + endtime复合索引(名为site_endtime):

mysql> explain select count(*) from `deal_expired` where `site`=8&&`area`=122 && endti
me<1310444996056;
+--------+------+-------------------------------+--------------+---------+------
-+------+-------------+
| table  | type | possible_keys                 | key          | key_len | ref
 | rows | Extra       |
+--------+------+-------------------------------+--------------+---------+------
-+------+-------------+
| deal_expired | ref  | name,url,endtime,site_endtime | site_endtime |       4 | const
 |  353 | Using where |
+--------+------+-------------------------------+--------------+---------+------
-+------+-------------+
1 row in set (0.00 sec)

Answer 1

第一个查询应该使用SQL_NO_CACHE告诉MySQL不要将结果放入缓存中.第二个查询使用缓存并告诉MySQL不要缓存该查询的结果,该查询不执行任何操作.

tl; dr - 撤消您的查询.

Answer 2

"如何获得相同的SQL运行时？"的答案.是 - 你不能.如果您的查询读取某些行,则它们将被缓存,具体取决于所使用的存储引擎,这些行位于OS缓存(myisam)或缓冲池(innodb)中.如果缓存行,则第二次运行相同的查询要快得多,因为MySQL不必从磁盘读取.

Answer 3

我的印象是,包含在当前运行时计算的任何类型的SQL函数都不会缓存.你有没有试过像下面这样的事情？

select count(*), now() from `deal_expired` where `site`=8&&`area`=122 && endtime<1310444996056;