mysql查询缓存命中总是0

mysql查询缓存命中总是0
mysql> show variables like '%query_cache%';+------------------------------+----------+
| Variable_name                | Value    |
+------------------------------+----------+
| have_query_cache             | YES      | 
| query_cache_limit            | 1048576  | 
| query_cache_min_res_unit     | 4096     | 
| query_cache_size             | 16777216 | 
| query_cache_type             | ON       | 
| query_cache_wlock_invalidate | OFF      | 
+------------------------------+----------+
mysql> select count(*) from user;
+----------+
| count(*) |
+----------+
|       17 | 
+----------+0条命中
mysql> show status like 'Qcache_hits';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Qcache_hits   | 0     | 
+---------------+-------+//没有被缓存的查询反而增加
mysql> show status like 'Qcache_not_cached';
+-------------------+-------+
| Variable_name     | Value |
+-------------------+-------+
| Qcache_not_cached | 166   | 
+-------------------+-------+