mysql的query_cache 相當於 oracle 裏的result cache
mysql> mhow variables like '%query_cache%';
+------------------------------+---------+
| Variable_name | Value |
+------------------------------+---------+
| have_query_cache | YES |
| query_cache_limit | 1048576 |
| query_cache_min_res_unit | 4096 |
| query_cache_size | 1048576 |
| query_cache_type | DEMAND |
| query_cache_wlock_invalidate | OFF |
+------------------------------+---------+
6 rows in set (0.01 sec)
設置爲DEMAND後，可以加hint手動開啓
不加hint，可以看到多次查詢速度相差不多
mysql> select count(*) from t100 a,t100 b,t100 c,emp e,t10;
+----------+
| count(*) |
+----------+
| 28753060 |
+----------+
1 row in set (1.98 sec)
mysql> select count(*) from t100 a,t100 b,t100 c,emp e,t10;
+----------+
| count(*) |
+----------+
| 28753060 |
+----------+
1 row in set (1.98 sec)而增加hint後，第二次查詢幾乎不需要時間
mysql> select sql_cache count(*) from t100 a,t100 b,t100 c,emp e,t10;
+----------+
| count(*) |
+----------+
| 28753060 |
+----------+
1 row in set (2.02 sec)
mysql> select sql_cache count(*) from t100 a,t100 b,t100 c,emp e,t10;
+----------+
| count(*) |
+----------+
| 28753060 |
+----------+
1 row in set (0.00 sec)