费时费力的MySQL性能问题

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费时费力的MySQL性能问题

诡异的性能问题，还在同一个库下，两张表，相同表结构和行数，update和insert居然能出现性能差异。但是count无性能查询。

mysql> update t1 set inventory_id=888,customer_id=888 where rental_id=15973;
Query OK, 0 rows affected (0.03 sec)
Rows matched: 1  Changed: 0  Warnings: 0

mysql> update rental set inventory_id=888,customer_id=888 where rental_id=15973;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

mysql> select count(*) from t1;
+----------+
| count(*) |
+----------+
|    16044 |
+----------+
1 row in set (0.00 sec)

mysql> select count(*) from rental;
+----------+
| count(*) |
+----------+
|    16044 |
+----------+
1 row in set (0.00 sec)

mysql> update t1 set inventory_id=888,customer_id=888 where rental_id=15973;
Query OK, 0 rows affected (0.03 sec)
Rows matched: 1  Changed: 0  Warnings: 0

mysql> update rental set inventory_id=888,customer_id=888 where rental_id=15973;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0