I wouldn’t say it’s common knowledge that you should look at execution plans when tuning queries, but it’s not exactly uncommon knowledge either. But, people tend to get focused on just looking at the graphical part of the plan and there’s just not enough information there. Let’s take a look at a query:
FROM Production.Product AS p
LEFT JOIN Production.ProductReview AS pr
ON pr.ProductID = p.ProductID
JOIN Production.ProductSubcategory AS ps
ON p.ProductSubcategoryID = ps.ProductSubcategoryID
JOIN Production.ProductCategory AS pc
ON pc.ProductCategoryID = ps.ProductCategoryID
JOIN Purchasing.ProductVendor AS pv
JOIN Purchasing.Vendor AS v
ON v.BusinessEntityID = pv.BusinessEntityID
ON pv.ProductID = p.ProductID
WHERE v.ModifiedDate = '2006-02-17 00:00:00.000'
AND p.Color LIKE 'Y%';
This generates an execution plan that looks like this:
Neither v.ModifiedDate nor p.Color have indexes. Yet, we only see a single scan in this plan, on the BusinessEntity.Vendor table. Why? Well, we can’t tell from the GUI directly, so, you have to look to the tool tips or the properties. The tool tip in this case actually proves helpful, as does the properties:
In short, the clustered index is used to seek out a number of rows and then a secondary predicate is placed on those few rows to further filter the results. You would never even guess at that using just the GUI alone. You have to look to the details of the execution plan to understand that. There are lots of other examples where you can only get the information from the properties because not everything is available in the tooltip. This can include things like Optimization Level, Reason For Early Termination, Scan Direction, and a bunch of other things. Just remember to drill down to the properties in order to better understand your execution plans.
Why do you need information like this? Are we going to tune this query? Maybe we could help it by adding an index t the Vendor table. But, at least for this query with this date range, it appears we don’t need an index on the Product table. But, that may change depending on the number of rows returned from the Vendor table. A different data set can result in an entirely different execution plan with entirely different performance characteristics.
Query tuning is tough. That’s why I’ve put together a one-day seminar to get you started on it. There’s still room in Albany, on July 25th. You can register here. I’ll also be teaching at SQL Connections in September in Las Vegas. Go here to register for this great event. In Belgium in October, I’ll be doing an all day session on just execution plans at SQL Server Days. Go here to register for this event. When I did a similar session at the PASS Summit two years ago, it sold out. I’ll be presenting at Summit this year in Seattle in November. Go here to register. But do it early or you may be put on a waiting list.