order by rand()优化

优化前：

SELECT
    id,
    loan_id,
    NAME,
    company
FROM
    tablename
WHERE
    time BETWEEN 1522512000
AND 1525103999
AND loan_time BETWEEN 1522512000
AND 1525103999
AND STATUS = 1
AND is_again_yd = 0
AND company <> 1
order by rand()
LIMIT 30

优化方案一：

采用JOIN，查询max(id) * rand()来随机获取数据。
SELECT
    t1.id,
    t1.loan_id,
    t1. NAME,
t1.company
FROM
    `tablename` AS t1
JOIN (
    SELECT
        ROUND(
            RAND() * (
                SELECT
                    MAX(id)
                FROM
                    `tablename`
            )
        ) AS id
) AS t2
WHERE
    t1.id >= t2.id
AND time BETWEEN 1522512000
AND 1525103999
AND loan_time BETWEEN 1522512000
AND 1525103999
AND STATUS = 1
AND is_again_yd = 0
AND company <> 1
ORDER BY t1.id ASC
LIMIT 30;

优化方案二：

 SELECT
   t1.id,
   t1.loan_id,
   t1.NAME
 FROM
   tablename t1
 WHERE
 t1.id >= (
         SELECT
             floor(
                 RAND() * (
                     SELECT
                         MAX(id)
                     FROM
                         `tablename`
                 )
             )
     )
 AND t1.time BETWEEN 1522512000 AND 1525103999
 AND t1.STATUS IN (1, 2, 4)
 AND t1.type IN (1, 2)
 AND t1.belong_company = 0
 AND t1.is_del = 0
 AND NOT EXISTS (
 SELECT
 loan_id
 FROM
 tablename2 t2
 WHERE
 t2.loan_id = t1.loan_id
 AND t2.company = 1
 )
 ORDER BY id
 limit 20

说明：这种方案替代mysql rand(),性能太幅度提升，随机性没有rand()强，如果随机性强的请选择替代方案。