Mysql query with Left Join is too very slow

Question

Query:

   select `r`.`id` as `id` 
     from `tbl_rls` as `r` 
left join `tblc_comment_manager` as `cm` on `cm`.`rlsc_id` != `r`.`id`

Both tabl

Answer 1

You may need to provide more info. But one thing I would try is reversing the order of your ON clause (because it's so easy):

ON r.id != cm.rlsc_id

Edit: and you should put indexes on your PK (id) columns.

But I think this article might help you out.

Basically it says that NOT IN takes less resources than LEFT JOIN. A commenter in that article mentions using NOT EXISTS is best.

Also, I'm not sure this is accurate or not, but this article says that NOT IN does a full table scan, and NOT EXISTS can use an index.