Hi folks,
Consider I have the following table named orders
OrderID / CustID / ProductID / orderDate / storeID / completed
I want to get the clients who ordered more than one product in some stores. Here's the query I'm using
Select custID from orders where storeID in (1,2,3) and completed = 1 group by custID having count(distinct ProductID) > 1
The table needs to be MyISAM and when I look at the explain output I get no candidate indexes.
What indexes should I create to get fast response or what would be the alternate fastest method?
Thanks in advance.
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- Since you are in "tuning" mood, you may experiment with as many indexes as you feel might be useful. And of course remove unneeded indexes when coming back to "production" mood.
- About the various indexes suggested: I would probably experiment with those in versions 'single' as 'coumpound'. For instance, a coumpound index made of (storeID, completed, custID, productsID) in that order, but again you may want to experiment with different orders.
NOTE: although EXPLAIN gives you useful hints about the strategy that MySQL plans to use, you also need to run and time these queries. EXPLAIN is a guess made from the way your data is defined (and which indexs are available), there might be huge discrepancies with running your real data
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by: yessirnosirPosted on 2007-08-29 at 18:25:29ID: 19796847
Right now does your "Explain" output show type as "ALL", i.e. that the query must read ALL the rows of your table? I think if you add indexes for the columns in your WHERE clause, you can reduce that substantially. If you index StoreID then the query would only look at the rows from those three stores. If you additionally index Completed, the query narrows it down to only completed orders at those stores. I'm not sure if there is any benefit in indexing product ID in the having clause. But one could easily imagine doing other queries on particular products, customers, orders, or stores, so in the absence of any other info about what you're trying to do, I'd probably put an index on all four of those columns. That would marginally slow down Inserts, but would greatly speed up Selects.
Here's a pretty good discussion of MySQL indexes http://www.databasejournal