問題描述

看到網(wǎng)上的資料說(shuō)：

in 和 not in 也要慎用，否則會(huì)導(dǎo)致全表掃描，如：select id from t where num in(1,2,3)對(duì)于連續(xù)的數(shù)值，能用 between 就不要用 in 了：select id from t where num between 1 and 3

問題解答

連續(xù)數(shù)值當(dāng)然between好了 減少解析 并且in的范圍默認(rèn)超過一定數(shù)目就會(huì)走全表 9個(gè)還是多少忘了

針對(duì)樓下評(píng)論再補(bǔ)充一下答案：in走全表是分情況的，上面的解答只是憑記憶大概說(shuō)了一下，樓下小伙伴比較認(rèn)真，這里就再解釋一下，這個(gè)不是個(gè)數(shù)而應(yīng)該是個(gè)比例，大概25%-35%左右，你要再問到底多少不好意思水平有限不讀源碼沒辦法確認(rèn)。然后這個(gè)30左右的比例也并不是說(shuō)一定走全表掃描，因?yàn)閙ysql還有一個(gè)索引掃描，就是說(shuō)如果select的內(nèi)容在你的索引里面就能找到的話當(dāng)然不會(huì)去掃全表了，比如下面的例子中select id from ttt where id in (..);和select * from ttt where id in (...);前面這個(gè)肯定是走主鍵掃描，即使你in了所有id值他也是走主鍵，而后面的情況就是這種百分比的情況了，具體看下面示例，歡迎指正^_^額，然后還想說(shuō)一下between的情況，為什么好是因?yàn)槌怂饕紊线B續(xù)存取減少解析以外，還有一個(gè)情況就是在磁盤尋址檢索數(shù)據(jù)的時(shí)候，會(huì)默認(rèn)讀取第一次取值附近的部分?jǐn)?shù)據(jù)（有這么一個(gè)概率算法說(shuō)的是當(dāng)一個(gè)數(shù)據(jù)被檢索到的時(shí)候，他附近的數(shù)據(jù)也很大概率會(huì)被用到）所以就有了這么一個(gè)一次性取出冗余數(shù)據(jù)避免多次尋址的情況，這時(shí)候使用between的連續(xù)取值就正適用了

mysql> select * from ttt;+----+-------------------+| id | name |+----+-------------------+| 1 | I17021234001 || 2 | IC17031234002 || 3 | C17041234003 || 4 | IAsEw1234001 || 5 | I17021234001A2 || 6 | IC17031234002A2 || 7 | C17041234003A2 || 8 | IAsEw1234001A2 || 9 | I17021234001A2 || 10 | IC17031234002A2 || 11 | C17041234003A2 || 12 | IAsEw1234001A2 || 13 | I17021234001A2A2 || 14 | IC17031234002A2A2 || 15 | C17041234003A2A2 || 16 | IAsEw1234001A2A2 || 17 | I17021234001A2 || 18 | IC17031234002A2 || 19 | C17041234003A2 || 20 | IAsEw1234001A2 |+----+-------------------+20 rows in set (0.00 sec)mysql> show create table ttt;+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| Table | Create Table |+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| ttt | CREATE TABLE `ttt` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` char(32) DEFAULT NULL, PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=32 DEFAULT CHARSET=utf8 |+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+1 row in set (0.00 sec)mysql> explain select * from ttt where id in (1,2,3,4,5,6);+----+-------------+-------+------+---------------+------+---------+------+------+-------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+------+---------------+------+---------+------+------+-------------+| 1 | SIMPLE | ttt | ALL | PRIMARY | NULL | NULL | NULL | 20 | Using where |+----+-------------+-------+------+---------------+------+---------+------+------+-------------+1 row in set (0.00 sec)mysql> explain select * from ttt where id in (1,2,3,4,5);+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+| 1 | SIMPLE | ttt | range | PRIMARY | PRIMARY | 4 | NULL | 5 | Using where |+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+1 row in set (0.00 sec)mysql> explain select * from ttt where id in (1,2,3);+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+| 1 | SIMPLE | ttt | range | PRIMARY | PRIMARY | 4 | NULL | 3 | Using where |+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+1 row in set (0.00 sec)mysql> explain select * from ttt where id in (1,2,3,4,5,6,7);+----+-------------+-------+------+---------------+------+---------+------+------+-------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+------+---------------+------+---------+------+------+-------------+| 1 | SIMPLE | ttt | ALL | PRIMARY | NULL | NULL | NULL | 20 | Using where |+----+-------------+-------+------+---------------+------+---------+------+------+-------------+1 row in set (0.00 sec)mysql> explain select id from ttt where id in (1,2,3,4,5,6,7);+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+| 1 | SIMPLE | ttt | index | PRIMARY | PRIMARY | 4 | NULL | 20 | Using where; Using index |+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+1 row in set (0.00 sec)mysql> explain select name from ttt where id in (1,2,3,4,5,6,7);+----+-------------+-------+------+---------------+------+---------+------+------+-------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+------+---------------+------+---------+------+------+-------------+| 1 | SIMPLE | ttt | ALL | PRIMARY | NULL | NULL | NULL | 20 | Using where |+----+-------------+-------+------+---------------+------+---------+------+------+-------------+1 row in set (0.00 sec)回答2：

根據(jù)數(shù)據(jù)庫(kù)中B樹索引的存儲(chǔ)結(jié)構(gòu)，在葉節(jié)點(diǎn)存儲(chǔ)指向數(shù)據(jù)的物理地址，而這個(gè)物理地址當(dāng)存在聚簇索引時(shí)是有序的。

如果是連續(xù)數(shù)值，between在找到第一個(gè)匹配值后，則直接從該地址往后搜索，直到最后一個(gè)元素為止。這樣就不會(huì)對(duì)后續(xù)值進(jìn)行索引掃描，因此速度快了。對(duì)于in操作，不大清楚，但是估計(jì)應(yīng)該會(huì)對(duì)全索引進(jìn)行掃描吧。回答3：

EXPLAIN mysql 語(yǔ)句 看下輸出

使用between時(shí)只需要匹配上下界，故而會(huì)快一點(diǎn)；in每個(gè)都要看一遍，會(huì)造成全表掃描。