DBA

TSQL Tuesday #83 – The Stats update solution

TSQL Tuesday is a monthly blog part hosted by a different blogger every month – it was started by Adam Machanic. This week’s TSQL Tuesday is hosted by Andy Mallon – the topic is ‘We’re dealing with the same problem’. I have chosen to write about a common problem I have encountered at many places I have worked at. The problem has to do with queries or stored procedures that suddenly start to perform poorly – when no changes have been made to the code or to the server.

The common perception/misunderstanding I have encountered for this is that it is only an issue with statistics updates and updating statistics with full scan should take care of it. In many cases this is the real reason. In some cases it really isn’t. It could be an issue with parameter sniffing, and a plan being reused that was generated for a set of parameters and appropriate for that set of parameters. But, most people jump to fixing statistics. This is especially true when they don’t really have the runtime plan it used, can’t find it in the cache and are just going with some sort of past experience.

At one place I was at people would update statistics every 10 minutes or so in a frantic attempt to ‘fix the slow query’, which would at many times not respond at all. At another place they actually had an automated check for when the stored procedure finished running and if it was still running beyond its normal duration a full scan statistics update would fire off. None of these are wrong – but repeatedly doing the same thing when the query does not improve, and assuming that is the only reason for the problem is wrong.

What I do is to recompile the plan with different parameters – test it to see the performance. If you are able to get a difference in performance with a different set of parameters then it is probably a parameter sniffing issue. And, if the stats updates do not fix it it is probably that too. Statistics updates are never really a bad thing to do, but they may not fix every slow query there is. Check if the issue is with parameter sniffing also. Also make sure there are no changes that went out – to the code and to the environment, that may be contributing to it.

 

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