为了测试sql语句的效率，有时候要不用缓存来查询。
使用
SELECT SQL_NO_CACHE ...
语法即可
SQL_NO_CACHE的真正作用是禁止缓存查询结果，但并不意味着cache不作为结果返回给query。
目前流传的SQL_NO_CACHE不外乎两种解释：
1.对当前query不使用数据库已有缓存来查询，则当前query花费时间会多点
2.对当前query的产生的结果集不缓存至系统query cache里，则下次相同query花费时间会多点
我做了下实验，似乎两种都对。
sql_cache意思是说，查询的时候使用缓存。
对SQL_NO_CACHE的解释及测试如下:
SQL_NO_CACHE means that the query result is not cached. It does not mean
that the cache is not used to answer the query.
You may use RESET QUERY CACHE to remove all queries from the cache and
then your next query should be slow again. Same effect if you change
the table, because this makes all cached queries invalid.
mysql> select count(*) from users where email = 'hello';
+----------+
| count(*) |
+----------+
| 0 |
+----------+
1 row in set (7.22 sec)
mysql> select count(*) from users where email = 'hello';
+----------+
| count(*) |
+----------+
| 0 |
+----------+
1 row in set (0.45 sec)
mysql> select count(*) from users where email = 'hello';
+----------+
| count(*) |
+----------+
| 0 |
+----------+
1 row in set (0.45 sec)
mysql> select SQL_NO_CACHE count(*) from users where email = 'hello';
+----------+
| count(*) |
+----------+
| 0 |
+----------+
1 row in set (0.43 sec)
