Database changed

    mysql> select count(*) from test;

    +----------+

    | count(*) |

    +----------+

    | 2097152 |

    +----------+

    1 row in set (0.00 sec)

    +--------+------+

    | a | b |

    +--------+------+

    | 765909 | |

    +--------+------+

    1 row in set (8.78 sec) --用了将近9s

    You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times.也就是 order by rand()会去扫描多次，造成性能的下降。

    先取这个表的最大值*rand()，取出这个随机值后，在进行比对。

    mysql> select * from test

     -> where a >= (select floor(rand() * (select max(a) from test))) order by a limit 1;

    +-------+------+

    | a | b |

    +-------+------+

    | 99275 | |

    +-------+------+

    1 row in set (0.00 sec) --在0.00s左右