mysql trace

mysql> show variables like "%trace%";
mysql> set optimizer_trace="enabled=on",end_markers_in_json=on;
Query OK, 0 rows affected (0.00 sec)
mysql> set optimizer_trace_max_mem_size=1000000;
Query OK, 0 rows affected (0.00 sec)
mysql> select rental_id from rental where 1=1 and rental_date >= '2005-05-25 04:00:00' and rental_date <='2005-05-25 05:00:00' and inventory_id=4466;
+-----------+
| rental_id |
+-----------+
| 39 |
+-----------+
1 row in set (0.00 sec)

mysql>
mysql> select * from information_schema.optimizer_trace\G

mysql> alter table rental add index idx_rental_date (rental_date,inventory_id,customer_id);
Query OK, 0 rows affected (0.48 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql>
查看索引使用的好坏：

mysql> show status like "Handler_read%";
+-----------------------+-------+
| Variable_name | Value |
+-----------------------+-------+
| Handler_read_first | 3 |
| Handler_read_key | 8 |
| Handler_read_last | 0 |
| Handler_read_next | 16059 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 0 |
| Handler_read_rnd_next | 16337 |
+-----------------------+-------+
7 rows in set (0.00 sec)

mysql