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。