Question

I’m trying to learn something about Optimizing and indexes because I ran a insert select-query that required 4 min to complete. Now, I’ve added multiple indexes and it seems to have made my query run in 0.160sec. Now what I’m wondering is why the customer table is getting the using filesort message when i’m ordering by orderdate in my order table. Query and explain:

I’ve even tried an index in O(Orders) for (orderdate, orderid) and (orderdate, orderid, customerid). I thought one of them would help, but no dice. Can anyone help me understand why?

Answer 1

There is nothing wrong with having a query that uses “filesort”; all that means is that the results can’t be sorted based on an index.

Now the reason why the sort can’t be performed on an index is in this case because your ORDER BY contains columns from tables other than the first table in the join queue.

Since your query result doesn’t contain very many rows, the temporary table being used is probably in memory.

What happens is as the query results are fetched from that query is that the results are put into a temporary table so they can later be sorted.

Adding the initial indexes sped up your query most likely because MySQL was doing a full table scan to fetch the results initially which was very time consuming. Once you added the proper indexes, finding the records is extremely quick. It probably had to do a filesort on a temporary table originally but this was likely no slower or faster than it is now.

If you try moving the join for the Orders table and put it before the join of the Products table, you may be able to eliminate the use of the temporary table and file sort.

Check out what does using filesort mean? and How MySQL Uses Internal Temporary Tables for more information.