[mysql] 随机查询 效率比较

select
    primary_count as primaryCount,
    primary_score as primaryScore,
    junior_count as juniorCount,
    junior_score as juniorScore,
    senior_count as seniorCount,
    senoir_score as senoirScore,
    total_score as totalScore,
    pass_score as passScore
from pd_paper p
where p.is_valid = ''
order by RAND() limit 1

分析：
ORDER BY从句里面不能使用RAND()函数，因为这样会导致数据列被多次扫描。
测试发现这样效率非常低。一个15万余条的库，查询5条数据，要8秒以上。

You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY
would evaluate the column multiple times.

更高效的做法：查询max(id) * rand()来随机获取数据。

SELECT *
FROM `table` AS t1
JOIN (
    SELECT ROUND(RAND()
    * (SELECT MAX(id) FROM `table`)) AS id
) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id ASC LIMIT 5;

但是这样会产生连续的5条记录。解决办法只能是每次查询一条，查询5次。
即便如此也值得，因为15万条的表，查询只需要0.01秒不到。
采用join 语法  可以实现真正的随机。

SELECT *
FROM `table`
WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table` )
ORDER BY id LIMIT 1;

把语句完善一下，加上MIN(id)的判断。
如果没有加上MIN(id)的判断，结果有一半的时间总是查询到表中的前面几行。
完整的语句：

①  where 子句

SELECT * FROM `table`
WHERE id >= (SELECT floor( RAND() * ((SELECT MAX(id) FROM `table`)-
 (SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`)))
ORDER BY id LIMIT 1;

② join

SELECT *
FROM `table` AS t1
JOIN (
    SELECT ROUND(RAND()
    * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`))
    +(SELECT MIN(id) FROM `table`)) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id LIMIT 1;

#随机查询一套考卷定义
SELECT
    p.primary_count as primaryCount,
    p.primary_score as primaryScore,
    p.junior_count as juniorCount,
    p.junior_score as juniorScore,
    p.senior_count as seniorCount,
    p.senoir_score as senoirScore,
    p.total_score   as totalScore,
    p.pass_score as passScore   

FROM
    pd_paper AS p
JOIN
(
    SELECT ROUND(
        RAND()
        *((SELECT MAX(id) FROM pd_paper)-(SELECT MIN(id) FROM pd_paper))
        +(SELECT MIN(id) FROM pd_paper)
    ) AS id
)
    AS p2
WHERE p.id >= p2.id
ORDER BY p.id LIMIT 1;

最后在程序对这两个语句进行分别查询10次，
前者花费时间 0.147433 秒
后者花费时间 0.015130 秒
看来采用JOIN的语法比直接在WHERE中使用函数效率还要高很多。

附加一个复杂sql：按题目类型（三种）随机查询全部考题信息

SELECT * from (
    select
        p.id as id,
        p.title as title,
        p.question as question,
        p.answer as answer,
        p.crt_time as crtTime
    from
        pd_problem p
    join
        (SELECT ROUND(RAND()
            * ((SELECT MAX(pp.id) FROM pd_problem pp)-(SELECT MIN(pp.id) FROM pd_problem pp))
            +(SELECT MIN(pp.id) FROM pd_problem pp)) AS pid) AS p2
    where p.id >= p2.pid
    and p.is_valid = ''
    and p.paper_type = ''
    and p.paper_class = ''
    order by p.id limit 5
 ) as t1
 union all
 SELECT * from (
    select
        p.id as id,
        p.title as title,
        p.question as question,
        p.answer as answer,
        p.crt_time as crtTime
    from
        pd_problem p
    join
        (SELECT ROUND(RAND()
            * ((SELECT MAX(pp.id) FROM pd_problem pp)-(SELECT MIN(pp.id) FROM pd_problem pp))
            +(SELECT MIN(pp.id) FROM pd_problem pp)) AS pid) AS p2
    where p.id >= p2.pid
    and p.is_valid = ''
    and p.paper_type = ''
    and p.paper_class = ''
    order by p.id limit 5
 ) as t2
 union all
 SELECT * from (
    select
        p.id as id,
        p.title as title,
        p.question as question,
        p.answer as answer,
        p.crt_time as crtTime
    from
        pd_problem p
    join
        (SELECT ROUND(RAND()
            * ((SELECT MAX(pp.id) FROM pd_problem pp)-(SELECT MIN(pp.id) FROM pd_problem pp))
            +(SELECT MIN(pp.id) FROM pd_problem pp)) AS pid) AS p2
    where p.id >= p2.pid
    and p.is_valid = ''
    and p.paper_type = ''
    and p.paper_class = ''
    order by p.id limit 5
 ) as t3