Home/ Questions/Q 8914927
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-06-15T04:51:06+00:00

Say I have an Order table that has 100+ columns and 1 million rows.

  • 0

Say I have an Order table that has 100+ columns and 1 million rows. It has a PK on OrderID and FK constraint StoreID –> Store.StoreID.

1) select * from 'Order' order by OrderID desc limit 10;

the above takes a few milliseconds.

2) select * from 'Order' o join 'Store' s on s.StoreID = o.StoreID order by OrderID desc limit 10;

this somehow can take up to many seconds. The more inner joins I add, slows it down further more.

3) select OrderID, column1 from 'Order' o join 'Store' s on s.StoreID = o.StoreID order by OrderID desc limit 10;

this seems to speed the execution up, by limiting the columns we select.

There are a few points that I dont understand here and would really appreciate it if anyone more knowledgeable with mysql (or rmdb query execution in general) can enlighten me.

Query 1 is fast since it’s just a reverse lookup by PK and DB only needs to return the first 10 rows it encountered.

I don’t see why Query 2 should take for ever. Shouldn’t the operation be the same? i.e. get the first 10 rows by PK and then join with other tables. Since there’s a FK constraint, it is guaranteed that the relationship will be satisfied. So DB doesn’t need to join more rows than necessary and then trim the result, right? Unless, FK constraint allows null FK? In which case I guess a left join would make this much faster than an inner join?

Lastly, I’m guess query 3 is simply faster because less columns are used in those unnecessary joins? But why would the query execution need the other columns while joining? Shouldn’t it just join using PKs first, and then get the columns for just the 10 rows?

Thanks!

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    My understanding is that the mysql engine applies limit after any join‘s happen.

    From http://dev.mysql.com/doc/refman/5.0/en/select.html, The HAVING clause is applied nearly last, just before items are sent to the client, with no optimization. (LIMIT is applied after HAVING.)

    EDIT: You could try using this query to take advantage of the PK speed.

    select * from (select * from 'Order' order by OrderID desc limit 10) o
    join 'Store' s on s.StoreID = o.StoreID;

    • 0