Mysql query with Left Join is too very slow

问题

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.

回答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.

回答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)

回答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

回答4:

MySQL's EXPLAIN might help you finding out what is going on.

回答5:

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.

回答6:

Looks like you are wanting the r.id values that are not in the tblc_comment_manager table.

Use a Not In

select r.id as id
from tbl_rls as r
where r.id not in (select distinct cm.rlsc_id from tblc_comment_manager as cm)

来源：https://stackoverflow.com/questions/3257575/mysql-query-with-left-join-is-too-very-slow