今天下午闲来无事，使用navicat对mysql的查询条件in进行执行计划调试。mysql版本5.0
首先表结构如下
-- Table "user_core" DDL
CREATE TABLE `user_core` (
`userId` int(11) NOT NULL,
`nickname` varchar(255) DEFAULT NULL COMMENT '昵称',
`realName` varchar(255) DEFAULT NULL COMMENT '真实改名',
`email` varchar(255) DEFAULT NULL COMMENT '用户email与user_email表中的email不同',
`gender` int(4) DEFAULT NULL COMMENT '性别(0:女， 1:男)',
`birthday` date DEFAULT NULL COMMENT '生日',
`phone` varchar(255) DEFAULT NULL COMMENT '电话',
`idNumber` varchar(18) DEFAULT NULL COMMENT '身份证',
`countKey` varchar(255) DEFAULT NULL COMMENT '计统url',
`urlKey` varchar(255) DEFAULT NULL COMMENT '户用url(hiyoucai_1)',
`birthplace` bigint(11) DEFAULT NULL COMMENT '出生地',
`location` bigint(11) DEFAULT NULL COMMENT '所在地',
`qq` varchar(255) DEFAULT NULL,
`school` varchar(255) DEFAULT NULL COMMENT '毕业学校',
`profession` varchar(255) DEFAULT NULL COMMENT '职业',
`introduction` varchar(5000) DEFAULT NULL COMMENT '简介,改成5000，modified by dsl on 201204071802',
`createTime` datetime DEFAULT NULL,
`editWorkerId` int(11) DEFAULT NULL,
`verifyWorkerId` int(11) DEFAULT NULL COMMENT '最后核审者Id',
`lastVerifyTime` datetime DEFAULT NULL COMMENT '最后审核时间',
`pinyin` varchar(255) DEFAULT NULL COMMENT '新加字段，added by dsl at 201203061723',
`editTime` datetime DEFAULT NULL COMMENT '增加最后编辑时间，用于方便编辑，added by dsl at 201203071625',
`state` int(11) DEFAULT '0' COMMENT '状态 0:未审核 1:审核通过 2:审核未通过3:删除 4:系统默认头像',
`usertype` int(4) DEFAULT '0' COMMENT '0：表示普通用户，为默认值;1表示虚拟帐号，2表示明星用户。added by dsl at 201204071758。;3表示iphone虚拟帐号',
`userSource` int(4) DEFAULT '-1' COMMENT '0：腾讯qq，1：新浪微博，10：iphone端QQ注册，11：iphone端sina微薄注册',
`userDataIntegrity` int(11) DEFAULT '0' COMMENT '用户资料完整度',
`isAlreadyVerifyEmail` int(2) DEFAULT '0' COMMENT '箱邮是否已认证 0:未认证 1:已认证',
`isAlreadyVerifyPhone` int(2) DEFAULT '0' COMMENT '手机是否已认证 0:未认证 1:已认证',
`isAlreadyVerifyIdNumber` int(2) DEFAULT '0' COMMENT '身份证是否已认证 0:未认证 1:已认证',
`company` varchar(255) DEFAULT NULL COMMENT '公司名称 add by lwt on 201207051450',
`channel` varchar(50) DEFAULT NULL COMMENT '渠道，added by dsl at 20120727',
`channelHost` varchar(200) DEFAULT NULL COMMENT '渠道host, added by dsl at 201207271713',
`ip` varchar(255) DEFAULT NULL COMMENT '注册IP add by longwentao on 20120815',
PRIMARY KEY (`userId`),
UNIQUE KEY `urlKey` (`urlKey`)
) ENGINE=MyISAM DEFAULT CHARSET=gbk;然后执行以下语句
mysql> explain select * from user_core where userId in(1,2,3,4);
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | user_core | range | PRIMARY | PRIMARY | 4 | NULL | 4 | Using where |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
1 row in set
mysql>key=PRIMARY,key_len=4,可见使用了主键userId作为索引，
所以网上所说的使用 in 不能使用索引的(至少5.0以上版本的都可以)，
都是扯蛋（估计是百度google一下，所以千万不要人云亦云，关键点最好自己证实下），
实践才是证明真理的唯一标准。
也有人说，当in里面的条件太多了，无法使用索引。
再看看下面这条
mysql> explain select * from user_core where userId in(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,
22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,
55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,
88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,
116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,
141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,
166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,
191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,
216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,
241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,
266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,
291,292,293,294,295,296,297,298,299,300,301,302,303,304,305,306,307,308,309,310,311,312,313,314,315,
316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,335,336,337,338,339,340,
341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,
366,367,368,369,370,371,372,373,374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390,
391,392,393,394,395,396,397,398,399,400,401,402,403,404,405,406,407,408,409,410,411,412,413,414,415,
416,417,418,419,420,421,422,423,424,425,426,427,428,429,430,431,432,433,434,435,436,437,438,439,440,
441,442,443,444,445,446,447,448,449,450,451,452,453,454,455,456,457,458,459,460,461,462,463,464,465,
466,467,468,469,470,471,472,473,474,475,476,477,478,479,480,481,482,483,484,485,486,487,488,489,490,
491,492,493,494,495,496,497,498,499,500,501,502,503,504,505,506,507,508,509,510,511,512,513,514,515,
516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,
541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,
566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,
591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,
616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,
641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,
666,667,668,669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,
691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,
716,717,718,719,720,721,722,723,724,725,726,727,728,729,730,731,732,733,734,735,736,737,738,739,740,
741,742,743,744,745,746,747,748,749,750,751,752,753,754,755,756,757,758,759,760,761,762,763,764,765,
766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,
791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,
816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838,839,840,
841,842,843,844,845,846,847,848,849,850,851,852,853,854,855,856,857,858,859,860,861,862,863,864,865,
866,867,868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883,884,885,886,887,888,889,890,
891,892,893,894,895,896,897,898,899,900,901,902,903,904,905,906,907,908,909,910,911,912,913,914,915,
916,917,918,919,920,921,922,923,924,925,926,927,928,929,930,931,932,933,934,935,936,937,938,939,940,
941,942,943,944,945,946,947,948,949,950,951,952,953,954,955,956,957,958,959,960,961,962,963,964,965,
966,967,968,969,970,971,972,973,974,975,976,977,978,979,980,981,982,983,984,985,986,987,988,989,990,
991,992,993,994,995,996,997,998,999,1000);
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | user_core | range | PRIMARY | PRIMARY | 4 | NULL | 1000 | Using where |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
1 row in set
mysql>同上，即使数据有1000条，也是使用到了主键索引。
但是，mysql毕竟还是不如oracle强大，当使用子查询作为结果集时，是无法使用到索引的
mysql> explain select * from user_core where userId in(select userId from user_core where userId<10);
+----+--------------------+-----------+-----------------+---------------+---------+---------+------+-------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+-----------+-----------------+---------------+---------+---------+------+-------+--------------------------+
| 1 | PRIMARY | user_core | ALL | NULL | NULL | NULL | NULL | 20789 | Using where |
| 2 | DEPENDENT SUBQUERY | user_core | unique_subquery | PRIMARY | PRIMARY | 4 | func | 1 | Using index; Using where |
+----+--------------------+-----------+-----------------+---------------+---------+---------+------+-------+--------------------------+
2 rows in set
mysql>可以发现子查询select userId from user_core where userId<10使用到了索引,而主查询的key=NULL，type=ALL，
即进行了全表扫描,所以mysql在涉及子查询时，要根据业务需求考虑是否要分开查询语句。
最后，现在的数据库版本会自动匹配最佳查询，比如上面的select * from user_core where userId in(1,....,1000);
当user_core表里面的数据只有一千多条的话，mysql会自动匹配全表扫描（因为这样更快，你想想，
你想想表就1000条数据，你都全查了，还用毛索引，岂不是浪费），
而当数据量足够大的话，比如说上万条，才会使用到索引。
来源：oschina
链接：https://my.oschina.net/u/736704/blog/79503