優化mysql的limit offset的例子

2020-01-18 23:50:13

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經常碰到的一個問題是limit的offset太高，如：limit 100000,20，這樣系統會查詢100020條，然后把前面的100000條都扔掉，這是開銷很大的操作，導致查詢很慢。假設所有分頁的頁面訪問頻率一樣，這樣的查詢平均掃描表的一半數據。優化的方法，要么限制訪問后面的頁數，要么提升高偏移的查詢效率。

一個簡單的優化辦法是使用覆蓋查詢（covering index）查詢，然后再跟全行的做join操作。如：

復制代碼代碼如下:

SQL>select * from user_order_info limit 1000000,5;

這條語句就可以優化為：

復制代碼代碼如下:

select * from user_order_info inner join (select pin from user_order_info limit 1000000,5) as lim using(pin);
SQL>explain select * from user_order_info limit 1000000,5;
+----+-------------+-----------------+------+---------------+------+---------+------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------------+------+---------------+------+---------+------+----------+-------+
| 1 | SIMPLE | user_order_info | ALL | NULL | NULL | NULL | NULL | 23131886 | |
+----+-------------+-----------------+------+---------------+------+---------+------+----------+-------+
1 row in set (0.00 sec)
SQL>explain extended select * from user_order_info inner join (select pin from user_order_info limit 1000000,5) as lim using(pin);
+----+-------------+-----------------+--------+---------------+---------+---------+---------+----------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------------+--------+---------------+---------+---------+---------+----------+----------+-------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 5 | 100.00 | |
| 1 | PRIMARY | user_order_info | eq_ref | PRIMARY | PRIMARY | 42 | lim.pin | 1 | 100.00 | |
| 2 | DERIVED | user_order_info | index | NULL | PRIMARY | 42 | NULL | 23131886 | 100.00 | Using index |
+----+-------------+-----------------+--------+---------------+---------+---------+---------+----------+----------+-------------+
3 rows in set, 1 warning (0.66 sec)