当我们想用SQL_NO_CACHE来禁止结果缓存时发现结果和我们的预期不一样，查询执行的结果仍然是缓存后的结果。其实，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。
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)