Question

Consider the following two queries:

select a.*, c.*
from account a
join customer c on a.customer_id = c.id
join import i on a.import_id = i.id
join import_bundle ib on i.import_bundle_id = ib.id

and

select a.*, c.*
from account a
join customer c on a.customer_id = c.id
join import i on a.import_id = i.id
join import_bundle ib on i.import_bundle_id = ib.id
where ib.id = 8

The first query is fast and the second one is super slow. Any idea why? I’m guessing I need an index or something but I don’t understand how indexes work. I’m using MySQL.

Here’s what happens if I do an EXPLAIN on the second query:

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra   
 1  SIMPLE  ib  const   PRIMARY     PRIMARY     8   const   1   Using index
 1  SIMPLE  c   ALL     PRIMARY         144858  
 1  SIMPLE  a   ref     fk_account_customer_id,fk_account_import_id     fk_account_customer_id  8   mcif.c.id   2   
 1  SIMPLE  i   eq_ref  PRIMARY,import_bundle_id    PRIMARY     8   mcif.a.import_id    1   Using where

I don’t know how to interpret that, though.

Edit: this is what I ended up using:

select a.*,
       c.*
  from account a
  join customer c on a.customer_id = c.id
  join (select id,
               import_bundle_id
          from import
         where import_bundle_id = 8) i on a.import_id = i.id
  join import_bundle ib on i.import_bundle_id = ib.id

Adding an index on import_bundle.id didn’t do anything.

Answer 1

• Regarding performance, in your query, you really need a.* and c.*?

• So, the use of an index does not improve enough. I’m not familiar with mysql but could you try a join with a subquery like this?

    select a.*, c.*
    from account a
    join customer c on a.customer_id = c.id
    join 
    (    
        SELECT id, import_bundle_id FROM import WHERE id = 8
    ) as i on a.import_id = i.id
    join import_bundle ib on i.import_bundle_id = ib.id
    where ib.id = 8

• Probably, the best index are: An index on import.import_bundle_id and another index on import_bundle.id.