第一次运行这个sql,需要39秒,再次运行增加SQL_NO_CACHE,好像没有生效:
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)
我尝试了多种方法,here
甚至重启mysql服务器或更改表名,但我仍然不能让39秒运行这条SQL
我换了另一个SQL,并在第一次运行时增加了SQL_NO_CACHE,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
不起作用
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(命名为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)
最佳答案
第一个查询应该使用 SQL_NO_CACHE 告诉 MySQL 不要将 结果 放入缓存。第二个查询使用缓存,告诉 MySQL 不要缓存该查询的结果,它什么也不做。
tl;dr - 反转您的查询。
关于mysql - SQL_NO_CACHE 不起作用,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/6666631/