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 tables have 8k records but why is it very slow, taking 2-3 minutes and more sometimes?

OMG , this query makes mysql server down. Will get back to you peoples in a second :(

All peoples those suggested Indexing the columns are all Correct. Yeh the query i wrote was silly and buggy. Thanks correcting me.

Answer 1

Consider also indexing your tables. We're running multiple left joins on a 1million+ record table that doesn't take more than a second or two to return results.

Answer 2

Do you really need the != or is it meant to be =?

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

This will select nearly the cartesian product of the 2 tables. (I guess around 60 million rows)

Edit: From the comment

yes it is " != " to match tbl_rls.id those are not in tblc_comment_manager

I think this is what you need if you want to use the outer join approach.

 select DISTINCT `r`.`id` as `id` from `tbl_rls` as `r` 
  left join `tblc_comment_manager` as `cm` 
on  `cm`.`rlsc_id`=`r`.`id
WHERE `cm`.`rlsc_id` IS NULL

Although my preference is usually

 select `r`.`id` as `id` 
 from `tbl_rls`
 as `r` 
 WHERE NOT EXISTS(
          SELECT * FROM `tblc_comment_manager` as `cm` 
          WHERE  `cm`.`rlsc_id`=`r`.`id)

Answer 3

What do you want to select?

Use this query if you want to find tbl_rls records that haven't matching records in other table

select `r`.`id`
from `tbl_rls` as `r` 
left join `tblc_comment_manager` as `cm` 
    on  `cm`.`rlsc_id`=`r`.`id
where `cm`.`rlsc_id` IS NULL