Mysql从表中删除多个随机行

Question

我现在有一张有604 000行的表.我想删掉4000个随机行,所以我的表只包含60万个条目.

会有快速的方法吗？

非常感谢.

Answer 1

从理论上讲,这将是随机和快速的.在实践中,它只会很快:

DELETE FROM tableX
LIMIT 4000

这将是随机但非常慢,有600K行:

DELETE FROM tableX
ORDER BY RAND() 
LIMIT 4000

这不是真正随机的(因为ID中通常存在间隙),它甚至可能不会删除正好4000行(但是当存在许多间隙时会少一些),但它可能比之前更快.

需要在子查询中进行额外的换行,因为从多个表中删除的语法不允许LIMIT:

DELETE td
FROM
      tableX AS td
  JOIN 
      ( SELECT t.id
        FROM
             tableX AS t 
          CROSS JOIN
              ( SELECT MAX(id) AS maxid 
                FROM tableX
              ) AS m
          JOIN
              ( SELECT RAND() AS rndm
                FROM tableX AS tr
                LIMIT 5000
              ) AS r
            ON 
               t.id = CEIL( rndm * maxid )
        LIMIT 4000
      ) AS x
      ON 
         x.id = td.id 

解释输出(子查询的输出,来自400K行表):

id           table        possible_keys  key_len     rows 
 select_type         type             key     ref         Extra
1 PRIMARY <derived2> system                             1
1 PRIMARY <derived3> ALL                             5000
1 PRIMARY t          eq_ref PRIMARY PRIMARY 4 func      1 Using where;Using index
3 DERIVED tr         index          PRIMARY 4      398681 Using index
2 DERIVED                                            Select tables optimized away