mysql> select * from abc_number_prop where number_id in (select number_id from abc_number_phone where phone = '82306839');
為了節省篇幅，省略了輸出內容，下同。
67 rows in set (12.00 sec)
只有67行數據返回，卻花了12秒，而系統中可能同時會有很多這樣的查詢，系統肯定扛不住。用desc看一下(注：explain也可)
mysql> desc select * from abc_number_prop where number_id in (select number_id from abc_number_phone where phone = '82306839');
+----+--------------------+------------------+--------+-----------------+-------+---------+------------+---------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------------+--------+-----------------+-------+---------+------------+---------+--------------------------+
| 1 | PRIMARY | abc_number_prop | ALL | NULL | NULL | NULL | NULL | 2679838 | Using where |
| 2 | DEPENDENT SUBQUERY | abc_number_phone | eq_ref | phone,number_id | phone | 70 | const,func | 1 | Using where; Using index |
+----+--------------------+------------------+--------+-----------------+-------+---------+------------+---------+--------------------------+
2 rows in set (0.00 sec)
從上面的信息可以看出，在執行此查詢時會掃描兩百多萬行，難道是沒有創建索引嗎，看一下
mysql> show index from abc_number_phone;
+------------------+------------+-------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+------------------+------------+-------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| abc_number_phone | 0 | PRIMARY | 1 | number_phone_id | A | 36879 | NULL | NULL | | BTREE | | |
| abc_number_phone | 0 | phone | 1 | phone | A | 36879 | NULL | NULL | | BTREE | | |
| abc_number_phone | 0 | phone | 2 | number_id | A | 36879 | NULL | NULL | | BTREE | | |
| abc_number_phone | 1 | number_id | 1 | number_id | A | 36879 | NULL | NULL | | BTREE | | |
| abc_number_phone | 1 | created_by | 1 | created_by | A | 36879 | NULL | NULL | | BTREE | | |
| abc_number_phone | 1 | modified_by | 1 | modified_by | A | 36879 | NULL | NULL | YES | BTREE | | |
+------------------+------------+-------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
6 rows in set (0.06 sec)
mysql> show index from abc_number_prop;
+-----------------+------------+-------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------------+------------+-------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| abc_number_prop | 0 | PRIMARY | 1 | number_prop_id | A | 311268 | NULL | NULL | | BTREE | | |
| abc_number_prop | 1 | number_id | 1 | number_id | A | 311268 | NULL | NULL | | BTREE | | |
| abc_number_prop | 1 | created_by | 1 | created_by | A | 311268 | NULL | NULL | | BTREE | | |
| abc_number_prop | 1 | modified_by | 1 | modified_by | A | 311268 | NULL | NULL | YES | BTREE | | |
+-----------------+------------+-------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.15 sec)
從上面的輸出可以看出，這兩張表在number_id字段上創建了索引的。
看看子查詢本身有沒有問題。
mysql> desc select number_id from abc_number_phone where phone = '82306839';
+----+-------------+------------------+------+---------------+-------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------------+------+---------------+-------+---------+-------+------+--------------------------+
| 1 | SIMPLE | abc_number_phone | ref | phone | phone | 66 | const | 6 | Using where; Using index |
+----+-------------+------------------+------+---------------+-------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)
沒有問題，只需要掃描幾行數據，索引起作用了。查詢出來看看
mysql> select number_id from abc_number_phone where phone = '82306839';
+-----------+
| number_id |
+-----------+
| 8585 |
| 10720 |
| 148644 |
| 151307 |
| 170691 |
| 221897 |
+-----------+
6 rows in set (0.00 sec)
直接把子查詢得到的數據放到上面的查詢中
mysql> select * from abc_number_prop where number_id in (8585, 10720, 148644, 151307, 170691, 221897);
67 rows in set (0.03 sec)
速度也快，看來MySQL在處理子查詢的時候是不夠好。我在MySQL 5.1.42 和 MySQL 5.5.19 都進行了嘗試，都有這個問題。
搜索了一下網絡，發現很多人都遇到過這個問題：
參考資料1：使用連接（JOIN）來代替子查詢(Sub-Queries) mysql優化系列記錄
http://blog.csdn.net/hongsejiaozhu/article/details/1876181
參考資料2：網站開發日記(14)-MYSQL子查詢和嵌套查詢優化
http://dodomail.iteye.com/blog/250199
根據網上這些資料的建議，改用join來試試。
修改前：select * from abc_number_prop where number_id in (select number_id from abc_number_phone where phone = '82306839');
修改后：select a.* from abc_number_prop a inner join abc_number_phone b on a.number_id = b.number_id where phone = '82306839';
mysql> select a.* from abc_number_prop a inner join abc_number_phone b on a.number_id = b.number_id where phone = '82306839';
67 rows in set (0.00 sec)
效果不錯，查詢所用時間幾乎為0。看一下MySQL是怎么執行這個查詢的
mysql> desc select a.* from abc_number_prop a inner join abc_number_phone b on a.number_id = b.number_id where phone = '82306839';
+----+-------------+-------+------+-----------------+-----------+---------+-----------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-----------------+-----------+---------+-----------------+------+--------------------------+
| 1 | SIMPLE | b | ref | phone,number_id | phone | 66 | const | 6 | Using where; Using index |
| 1 | SIMPLE | a | ref | number_id | number_id | 4 | eap.b.number_id | 3 | |
+----+-------------+-------+------+-----------------+-----------+---------+-----------------+------+--------------------------+
2 rows in set (0.00 sec)
小結：當子查詢速度慢時，可用JOIN來改寫一下該查詢來進行優化。
網上也有文章說，使用JOIN語句的查詢不一定總比使用子查詢的語句快。
參考資料3：改變了對Mysql子查詢的看法
http://hi.baidu.com/yzx110/blog/item/e694f536f92075360b55a92b.html
正好手頭有本《高性能MySQL》，翻閱了一下，第4.4節“MySQL查詢優化器的限制”4.4.1小節“關聯子查詢”正好講到這個問題。
MySQL有時優化子查詢很差，特別是在WHERE從句中的IN()子查詢。像上面我碰到的情況，其實我的想法是MySQL會把
select * from abc_number_prop where number_id in (select number_id from abc_number_phone where phone = '82306839');
變成下面的樣子
select * from abc_number_prop where number_id in (8585, 10720, 148644, 151307, 170691, 221897);
但不幸的是，實際情況正好相反。MySQL試圖讓它和外面的表產生聯系來“幫助”優化查詢，它認為下面的exists形式更有效率
select * from abc_number_prop where exists (select * from abc_number_phone where phone = '82306839' and number_id = abc_number_prop.number_id);
mysql> select * from abc_number_prop where exists (select * from abc_number_phone where phone = '82306839' and number_id = abc_number_prop.number_id);
67 rows in set (10.89 sec)
mysql> desc select * from abc_number_prop where exists (select * from abc_number_phone where phone = '82306839' and number_id = abc_number_prop.number_id);
+----+--------------------+------------------+--------+-----------------+-------+---------+-------------------------------------+---------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------------+--------+-----------------+-------+---------+-------------------------------------+---------+--------------------------+
| 1 | PRIMARY | abc_number_prop | ALL | NULL | NULL | NULL | NULL |2660707 | Using where |
| 2 | DEPENDENT SUBQUERY | abc_number_phone | eq_ref | phone,number_id | phone | 70 | const,eap.abc_number_prop.number_id | 1 | Using where; Using index |
+----+--------------------+------------------+--------+-----------------+-------+---------+-------------------------------------+---------+--------------------------+
2 rows in set (0.01 sec)
這種in子查詢的形式，在外部表（比如上面的abc_number_prop）數據量較大的時候效率是很差的。（如果對于較小的表，不會造成顯著地影響）
文中說到一種優化方式就是，手工將in里面的子查詢查詢出來，然后再拼裝執行 ，這在程序中是可行的。
posted on 2012-03-10 22:17
kxbin 閱讀(264)
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