Speed up sql JOIN

Question

First of all, some background.

We have an order processing system, where staff enter billing data about orders in an app that stores it in a sql server 2000 database. This database isn't the real billing system: it's just a holding location so that the records can be run into a mainframe system via a nightly batch process.

This batch process is a canned third party package provided by an outside vendor. Part of what it's supposed to do is provide a report for any records that were rejected. The reject report is worked manually.

Unfortunately, it turns out the third party software doesn't catch all the errors. We have separate processes that pull back the data from the mainframe into another table in the database and load the rejected charges into yet another table.

An audit process then runs to make sure everything that was originally entered by the staff can be accounted for somewhere. This audit takes the form of an sql query we run, and it looks something like this:

SELECT *
FROM [StaffEntry] s with (nolock)
LEFT JOIN [MainFrame] m with (nolock)
    ON m.ItemNumber = s.ItemNumber 
        AND m.Customer=s.Customer 
        AND m.CustomerPO = s.CustomerPO -- purchase order
        AND m.CustPORev = s.CustPORev  -- PO revision number
LEFT JOIN [Rejected] r with (nolock) ON r.OrderID = s.OrderID
WHERE s.EntryDate BETWEEN @StartDate AND @EndDate
    AND r.OrderID IS NULL AND m.MainFrameOrderID IS NULL

That's heavily modified, of course, but I believe the important parts are represented. The problem is that this query is starting to take too long to run, and I'm trying to figure out how to speed it up.

I'm pretty sure the problem is the JOIN from the StaffEntry table to the MainFrame table. Since both hold data for every order since the beginning of time (2003 in this system), they tend to be a little large. The OrderID and EntryDate values used in the StaffEntry table are not preserved when imported to the mainframe, which is why that join is a little more complicated. And finally, since I'm looking for records in the MainFrame table that don't exist, after doing the JOIN we have that ugly IS NULL in the where clause.

The StaffEntry table is indexed by EntryDate (clustered) and separately on Customer/PO/rev. MainFrame is indexed by customer and the mainframe charge number (clustered, this is needed for other systems) and separately by customer/PO/Rev. Rejected is not indexed at all, but it's small and testing shows it's not the problem.

So, I'm wondering if there is another (hopefully faster) way I can express that relationship?

Answer 1

First off, you can get rid of the second LEFT JOIN.

Your WHERE was removing out any matches, anyhow... For instance, if S.OrderID was 1 and there was a R.OrderID with a value of 1, the IS NULL enforcement in the WHERE wouldn't allow it. So it'll only return records where s.OrderID IS NULL, if I'm reading it correctly...

Secondly, if you're dealing with a large amount of data, adding on a NOLOCK table hint typically won't hurt. Assuming you don't mind the possibility of a dirty-read here or there :-P Usually worth the risk, though.

SELECT *
FROM [StaffEntry] s (nolock)
LEFT JOIN [MainFrame] m (nolock) ON m.ItemNumber = s.ItemNumber 
    AND m.Customer=s.Customer 
    AND m.CustomerPO = s.CustomerPO -- purchase order
    AND m.CustPORev = s.CustPORev  -- PO revision number
WHERE s.EntryDate BETWEEN @StartDate AND @EndDate
    AND s.OrderID IS NULL

Lastly, there was a part of your question which wasn't too clear for me...

"since I'm looking for records in the MainFrame table that don't exist, after doing the JOIN we have that ugly IS NULL in the where clause."

Ok... But are you trying to limit it to just where those MainFrame table records don't exist? If so, you'll want that expressed in the WHERE as well, right? So something like this...

SELECT *
FROM [StaffEntry] s (nolock)
LEFT JOIN [MainFrame] m (nolock) ON m.ItemNumber = s.ItemNumber 
    AND m.Customer=s.Customer 
    AND m.CustomerPO = s.CustomerPO -- purchase order
    AND m.CustPORev = s.CustPORev  -- PO revision number
WHERE s.EntryDate BETWEEN @StartDate AND @EndDate
    AND s.OrderID IS NULL AND m.ItemNumber IS NULL

If that's what you were intending with the original statement, perhaps you can get rid of the s.OrderID IS NULL check?

Answer 2

Before you even start looking at changing your query, you should ensure that all tables have a clustered index that makes sense for both this query and all other vital queries. Having clustered indexes on your tables i vital in sql server to ensure proper performance.