What to look for in Execution plans ?

Question

Hi Pals,
Assuming the query execution is slow , I have collected the xml plan. What should I look for in the actual execution plan . What are the top 10 things I need to watch out for?
I know this is a broad and generic question but I am looking for anyone who is experienced in query tuning to answer this question.
Thanks in Advance.

Answer 1

Reading execution plans is a bit of an art. But a couple of things:
1) Download and install SQL Sentry Plan Explorer (www.sqlsentry.net). This is a free tool that in msny cases gives a better experience to look at execution plans, particularly big and ugly ones.
2) I usually look for the thickest arrows, as thickness indicates the number of rows being processed.
3) Look for deviations between estimates and actual values, as these deviations are often the source for bad performance. In Plan Explorer, you can quickly flip between the too. In SSMS you need to look at the popup for every operator. (But again, it is
the operator with fat arrows that are of most interest - and those before them.
4) The way to read the plan is that the left-most operator asks the operators it is connected to for data. The net effect is that data flows from right to left, and the right-hand side if often more interesting.
5) Don't pay much attention to the percentages about operator cost. These percentages are estimates only,
not actual values. They are only reliable if the estimates are correct all the way through - and when you have a bad plan that is rarely the case.
This was the overall advice. Then there is more specific one: are indexes being used when expected? Note that scans are necessarily not bad. Sometimes your problem is that you have a loop join + index seek, when you should have had two scans and a hash join.
Try to get a grip of how you would process the query, if you had to do it manually. Does the plan match that idea?
Erland Sommarskog, SQL Server MVP, [email protected]