MySql 100万级别数据中随机获取一条或多条记录之RAND()优化
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处理业务中，有这样的需求，例如：有100W甚至更多的用户，此时我们要随机一条男性或者女性用户出来做数据操作。基于这个需求，我们做一下实验。
基础准备
- 准备一张用户表，结构如下
CREATE TABLE `user` ( `uid` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT '用户ID', `name` varchar(255) DEFAULT NULL COMMENT '用户姓名', `age` tinyint(3) unsigned DEFAULT 0 COMMENT '年龄', `gender` tinyint(3) unsigned DEFAULT 2 COMMENT '性别 2 人妖 1 男 0 女', `create_time` int(10) unsigned DEFAULT 0 COMMENT '创建时间', PRIMARY KEY (`uid`) ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8; - 简单写个MySQL储存
DELIMITER ;; USE `test`;; DROP PROCEDURE IF EXISTS `autoFull`;; CREATE DEFINER=`root`@`localhost` PROCEDURE`autoFull`(num INT) BEGIN #Routine body goes here... DECLARE count INT DEFAULT 0; DECLARE i INT DEFAULT 0; set @exesql = concat("insert into user(name,age,gender,create_time) values "); set @exedata = ""; while count<num do set @exedata = concat(@exedata, ",('",MD5(i), "','", floor(rand()*37 18), "','", ROUND(RAND() * 1), "','",current_timestamp(), "')"); set count=count 1; set i=i 1; if i00=0 then set @exedata = SUBSTRING(@exedata, 2); set @exesql = concat("insert into user(name,age,gender,create_time) values ", @exedata); prepare stmt from @exesql; execute stmt; DEALLOCATE prepare stmt; set @exedata = ""; end if; end while; if length(@exedata)>0 then set @exedata = SUBSTRING(@exedata, 2); set @exesql = concat("insert into user(name,age,gender,create_time) values ", @exedata); prepare stmt from @exesql; execute stmt; DEALLOCATE prepare stmt; end if; END;; DELIMITER ;
具体步骤：
mysql> use test
Database changed
mysql>
mysql> CREATE TABLE `user` (
-> `uid` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT '用户ID',
-> `name` varchar(255) DEFAULT NULL COMMENT '用户姓名',
-> `age` tinyint(3) unsigned DEFAULT 0 COMMENT '年龄',
-> `gender` tinyint(3) unsigned DEFAULT 2 COMMENT '性别 2 人妖 1 男 0 女',
-> `create_time` int(10) unsigned DEFAULT 0 COMMENT '创建时间',
-> PRIMARY KEY (`uid`)
-> ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
mysql> DELIMITER ;;
mysql>
mysql> DROP PROCEDURE IF EXISTS `autoFull`;;
Query OK, 0 rows affected (0.00 sec)
mysql>
mysql> CREATE DEFINER=`root`@`localhost` PROCEDURE`autoFull`(num INT)
-> BEGIN
-> #Routine body goes here...
-> DECLARE count INT DEFAULT 0;
-> DECLARE i INT DEFAULT 0;
-> set @exesql = concat("insert into user(name,age,gender,create_time) values ");
-> set @exedata = "";
-> while count<num do
-> set @exedata = concat(@exedata, ",('",MD5(i), "','", floor(rand()*37 18), "','", ROUND(RAND() * 1), "','",current_timestamp(), "')");
-> set count=count 1;
-> set i=i 1;
-> if i00=0
-> then
-> set @exedata = SUBSTRING(@exedata, 2);
-> set @exesql = concat("insert into user(name,age,gender,create_time) values ", @exedata);
-> prepare stmt from @exesql;
-> execute stmt;
-> DEALLOCATE prepare stmt;
-> set @exedata = "";
-> end if;
-> end while;
->
-> if length(@exedata)>0
-> then
-> set @exedata = SUBSTRING(@exedata, 2);
-> set @exesql = concat("insert into user(name,age,gender,create_time) values ", @exedata);
-> prepare stmt from @exesql;
-> execute stmt;
-> DEALLOCATE prepare stmt;
-> end if;
-> END;;
Query OK, 0 rows affected (0.00 sec)
mysql> DELIMITER ;//还原界定符
mysql> call autoFull(1000000);
Query OK, 0 rows affected, 64 warnings (1 min 3.81 sec)
//调用下储存查看下自己的储存
mysql> select count(uid) from user;
------------
| count(uid) |
------------
| 1000001 |
------------
1 row in set (0.20 sec)
mysql> 原始简单粗暴的 SQL 语句 select * from user order by RAND() LIMIT 1; (切勿使用)
mysql> select * from user order by RAND() LIMIT 1;
-------- ---------------------------------- ------ -------- -------------
| uid | name | age | gender | create_time |
-------- ---------------------------------- ------ -------- -------------
| 318393 | 48f8b305de34c87af8143fe1f24732ad | 24 | 0 | 2017 |
-------- ---------------------------------- ------ -------- -------------
1 row in set (17.19 sec)
mysql> 简单分析下：
mysql> explain select * from user order by RAND() LIMIT 1;
---- ------------- ------- ------ --------------- ------ --------- ------ --------- ---------------------------------
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
---- ------------- ------- ------ --------------- ------ --------- ------ --------- ---------------------------------
| 1 | SIMPLE | user | ALL | NULL | NULL | NULL | NULL | 1000340 | Using temporary; Using filesort |
---- ------------- ------- ------ --------------- ------ --------- ------ --------- ---------------------------------
1 row in set (0.00 sec)
type => all 呵呵，全表扫描 1000340 条数据
key => null 且没有索引，我们是随机查询
MySql 手册专门有提醒在 Order by 后面不能使用 RAND() 函数，会导致全表扫描简单优化 SQL 语句, 使用join
SELECT * FROM user AS u1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(uid) FROM `user`)-(SELECT MIN(uid) FROM user)) (SELECT MIN(uid) FROM user)) AS uid) AS u2 WHERE u1.uid >= u2.uid ORDER BY u1.uid LIMIT 1mysql> SELECT * FROM user AS u1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(uid) FROM `user`)-(SELECT MIN(uid) FROM user)) (SELECT MIN(uid) FROM user)) AS uid) AS u2 WHERE u1.uid >= u2.uid ORDER BY u1.uid LIMIT 1
-> ;
-------- ---------------------------------- ------ -------- ------------- --------
| uid | name | age | gender | create_time | uid |
-------- ---------------------------------- ------ -------- ------------- --------
| 798024 | c1a781e16b45f2ddc18b42365b8b0903 | 48 | 0 | 2017 | 798024 |
-------- ---------------------------------- ------ -------- ------------- --------
1 row in set (0.14 sec)以上测试发现快了不少，来分析下这个SQL语句，该SQL 语句的核心 Join和随机，随机的基本公式: RAND()*(max-min) mix,随机出一个 uid as u2 然后 条件查询，uid 自建索引，效率蛮高的。
mysql> explain SELECT * FROM user AS u1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(uid) FROM `user`)-(SELECT MIN(uid) FROM user)) (SELECT MIN(uid) FROM user)) AS uid) AS u2 WHERE u1.uid >= u2.uid ORDER BY u1.uid LIMIT 1;
---- ------------- ------------ -------- --------------- --------- --------- ------ -------- ------------------------------
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
---- ------------- ------------ -------- --------------- --------- --------- ------ -------- ------------------------------
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | |
| 1 | PRIMARY | u1 | range | PRIMARY | PRIMARY | 4 | NULL | 500170 | Using where |
| 2 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
| 5 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
| 4 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
| 3 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
---- ------------- ------------ -------- --------------- --------- --------- ------ -------- ------------------------------
6 rows in set (0.01 sec)如果想随机多条呢？修改LIMIT？来看看
mysql> SELECT * FROM user AS u1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(uid) FROM `user`)-(SELECT MIN(uid) FROM user)) (SELECT MIN(uid) FROM user)) AS uid) AS u2 WHERE u1.uid >= u2.uid ORDER BY u1.uid LIMIT 5;
mysql> SELECT * FROM user AS u1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(uid) FROM `user`)-(SELECT MIN(uid) FROM user)) (SELECT MIN(uid) FROM user)) AS uid) AS u2 WHERE u1.uid >= u2.uid ORDER BY u1.uid LIMIT 5;
-------- ---------------------------------- ------ -------- ------------- --------
| uid | name | age | gender | create_time | uid |
-------- ---------------------------------- ------ -------- ------------- --------
| 948535 | da2c5dbe42945a0cc5b46a1e6acf746b | 46 | 0 | 2017 | 948535 |
| 948536 | 8b6f00e7098d215d4a85b160fcbbce4f | 22 | 1 | 2017 | 948535 |
| 948537 | d3caa715182997a67fdf3ab245ea53f3 | 24 | 1 | 2017 | 948535 |
| 948538 | 727659b109bfe2a21f8be7a5c1d1b301 | 27 | 0 | 2017 | 948535 |
| 948539 | 4b5038af305fb4629d38d067f806c7ab | 27 | 0 | 2017 | 948535 |
-------- ---------------------------------- ------ -------- ------------- --------
5 rows in set (0.00 sec)如果使用以上的SQL语句，发现查询到的数据是连续的，我们要的是随机的，不难理解 LIMIT 5 得到当前查询条件的前五条，所以是相对连续的，uid 是自增的，因为用的是储存插入的，实际项目也是相对连续的。这条SQL 一次性查询无法达到我们的需求，则可分别一条条查询，如果要求的随机条数较多，那就不建议使用该条SQL语句了。
再来一条SQL语句
mysql> SELECT * FROM user WHERE uid >= ((SELECT MAX(uid) FROM user)-(SELECT MIN(uid) FROM user)) * RAND() (SELECT MIN(uid) FROM user) LIMIT 1
-> ;
------ ---------------------------------- ------ -------- -------------
| uid | name | age | gender | create_time |
------ ---------------------------------- ------ -------- -------------
| 2343 | c8dfece5cc68249206e4690fc4737a8d | 34 | 0 | 2017 |
------ ---------------------------------- ------ -------- -------------
1 row in set (0.01 sec)explain 下
mysql> explain SELECT * FROM user WHERE uid >= ((SELECT MAX(uid) FROM user)-(SELECT MIN(uid) FROM user)) * RAND() (SELECT MIN(uid) FROM user) LIMIT 1;
---- ------------- ------- ------ --------------- ------ --------- ------ --------- ------------------------------
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
---- ------------- ------- ------ --------------- ------ --------- ------ --------- ------------------------------
| 1 | PRIMARY | user | ALL | NULL | NULL | NULL | NULL | 1000340 | Using where |
| 4 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
| 3 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
| 2 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
---- ------------- ------- ------ --------------- ------ --------- ------ --------- ------------------------------
4 rows in set (0.00 sec)看看随机多条
mysql> SELECT * FROM user WHERE uid >= ((SELECT MAX(uid) FROM user)-(SELECT MIN(uid) FROM user)) * RAND() (SELECT MIN(uid) FROM user) limit 20;
------ ---------------------------------- ------ -------- -------------
| uid | name | age | gender | create_time |
------ ---------------------------------- ------ -------- -------------
| 786 | fc8001f834f6a5f0561080d134d53d29 | 24 | 0 | 2017 |
| 1961 | e4dd5528f7596dcdf871aa55cfccc53c | 54 | 1 | 2017 |
| 2958 | db5cea26ca37aa09e5365f3e7f5dd9eb | 27 | 0 | 2017 |
| 3122 | f231f2107df69eab0a3862d50018a9b2 | 43 | 1 | 2017 |
| 3445 | 12092a75caa75e4644fd2869f0b6c45a | 31 | 0 | 2017 |
| 4121 | 1b69ebedb522700034547abc5652ffac | 48 | 0 | 2017 |
| 4682 | 4f5c422f4d49a5a807eda27434231040 | 38 | 1 | 2017 |
| 4815 | 187acf7982f3c169b3075132380986e4 | 26 | 1 | 2017 |
| 5028 | f02208a057804ee16ac72ff4d3cec53b | 19 | 0 | 2017 |
| 5182 | fa3dade3a49305f27f64203452ac954c | 32 | 1 | 2017 |
| 5245 | b49d4455d64520060ac01fb5a3c757e4 | 34 | 1 | 2017 |
| 5405 | bb1443cc31d7396bf73e7858cea114e1 | 40 | 1 | 2017 |
| 5486 | 03b2ceb73723f8b53cd533e4fba898ee | 21 | 0 | 2017 |
| 5700 | 7f848746fe2599dc199a75f0d02fc3d6 | 36 | 0 | 2017 |
| 5835 | f5f3b8d720f34ebebceb7765e447268b | 36 | 0 | 2017 |
| 5991 | 1ae6464c6b5d51b363d7d96f97132c75 | 49 | 1 | 2017 |
| 6064 | 09ccf3183d9e90e5ae1f425d5f9b2c00 | 36 | 0 | 2017 |
| 6160 | 08ad21c6f9da6bdf51ae0b971f43d96d | 31 | 1 | 2017 |
| 6306 | ccf0304d099baecfbe7ff6844e1f6d91 | 48 | 1 | 2017 |
| 6810 | 6194a1ee187acd6606989f03769e8f7f | 41 | 0 | 2017 |
------ ---------------------------------- ------ -------- -------------
20 rows in set (0.01 sec)随机多条也是完美的，随机核心就是用 RAND() 随机出一个用户uid，或则随机区间，然后再进行limit 即可，此处基本阐述完了，欢迎大家批评指正。
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