DELETE using LEFT JOIN with LIMIT in MySQL

Question

I'm trying to delete orphan posts in my database and I have created this query:

DELETE post.*
      FROM foro_post AS post
      LEFT JOIN foro_thread AS thread USING(threadid)
      WHERE thread.threadid IS NULL

The problem is that I want to limit because my table has over 7,000,000 records.

As I can't use LIMIT with the query, I tried this and actually worked, but I'm not sure if is an efficient solution or if it could be done better.

DELETE post.*
      FROM foro_post AS post
      LEFT JOIN foro_thread AS thread USING(threadid)
      WHERE thread.threadid IS NULL
      AND post.postid < 500

      // Where < 500 should be increasing as I delete records

How can I do this more efficiently?

Answer 1

You can't use LIMIT directly within DELETE when you're referencing multiple tables at the same time, but you can get around that by encasing what you want to delete within a subselect:

DELETE po 
FROM   foro_post po
JOIN   (
       SELECT    p.postid
       FROM      foro_post p
       LEFT JOIN foro_thread t ON p.threadid = t.threadid
       WHERE     t.threadid IS NULL
       ORDER BY  p.postid
       LIMIT     50
       ) pp ON po.postid = pp.postid