慢SQL日誌裏看到一個三張表的關聯查詢，如下：
1 2 3 4 | SELECT COUNT(1) FROM refund_order_item i, artisan a, user u WHERE u.userid = i.user_id AND a.artisan_id = i.artisan_id; |
測試查詢時間：
1 2 3 4 5 6 7 8 9 10 | mysql> SELECT COUNT(1) -> FROM refund_order_item i, artisan a, user u -> WHERE u.userid = i.user_id -> AND a.artisan_id = i.artisan_id;+----------+| COUNT(1) |+----------+| 260605 |+----------+1 row in set (2.30 sec) |
查看執行計劃：
1 2 3 4 5 6 7 8 9 10 11 | mysql> explain SELECT COUNT(1) -> FROM refund_order_item i, artisan a, user u -> WHERE u.userid = i.user_id -> AND a.artisan_id = i.artisan_id;+----+-------------+-------+------------+--------+----------------------------+---------+---------+------------------+--------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+-------+------------+--------+----------------------------+---------+---------+------------------+--------+----------+-------------+| 1 | SIMPLE | i | NULL | ALL | idx_user_id,idx_artisan_id | NULL | NULL | NULL | 255599 | 100.00 | NULL || 1 | SIMPLE | a | NULL | eq_ref | PRIMARY | PRIMARY | 122 | hlj.i.artisan_id | 1 | 100.00 | Using index || 1 | SIMPLE | u | NULL | eq_ref | userid | userid | 122 | hlj.i.user_id | 1 | 100.00 | Using index |+----+-------------+-------+------------+--------+----------------------------+---------+---------+------------------+--------+----------+-------------+ |
可以看到refund_order_item表沒有走索引。
創建聯合索引：
1 | ALTER TABLE refund_order_item ADD INDEX idx_aid_uid (artisan_id, user_id); |
查看執行計劃：
1 2 3 4 5 6 7 8 | explain SELECT COUNT(1) FROM refund_order_item i, artisan a, user u WHERE u.userid = i.user_id AND a.artisan_id = i.artisan_id;+----+-------------+-------+------------+--------+----------------------------------------+-------------+---------+------------------+--------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+-------+------------+--------+----------------------------------------+-------------+---------+------------------+--------+----------+-------------+| 1 | SIMPLE | i | NULL | index | idx_user_id,idx_artisan_id,idx_aid_uid | idx_aid_uid | 244 | NULL | 255599 | 100.00 | Using index || 1 | SIMPLE | a | NULL | eq_ref | PRIMARY | PRIMARY | 122 | hlj.i.artisan_id | 1 | 100.00 | Using index || 1 | SIMPLE | u | NULL | eq_ref | userid | userid | 122 | hlj.i.user_id | 1 | 100.00 | Using index |+----+-------------+-------+------------+--------+----------------------------------------+-------------+---------+------------------+--------+----------+-------------+ |
可以看到執行計劃已經走索引。
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測試查詢時間：
1 2 3 4 5 6 7 8 9 10 | mysql> SELECT COUNT(1) -> FROM refund_order_item i, artisan a, user u -> WHERE u.userid = i.user_id -> AND a.artisan_id = i.artisan_id;+----------+| COUNT(1) |+----------+| 260605 |+----------+1 row in set (1.15 sec) |