Question

I now have a table that has 604 000 row. I would like to drop 4000 random rows so my table will only contains 600 000 entries.

Would there be a quick way to do so ?

Thanks a lot.

Answer 1

In theory, this will be random and fast. In practise, it will be only fast:

DELETE FROM tableX
LIMIT 4000

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This will be random but terribly slow, with 600K rows:

DELETE FROM tableX
ORDER BY RAND() 
LIMIT 4000

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This won’t be truly random (as there are usually gaps in the ids) and it may not even delete exactly 4000 rows (but a few less when there are many gaps) but it’s probably faster than the previous.

An extra wrapping in a subquery is needed because the syntax for Delete from multiple tables does not allow 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 

Explain output (of the subquery, from a 400K rows table):

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