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Reasons Your SQL Server Query Performance Fluctuates

Aaron Bertrand starts the count:

Query performance can fluctuate over time, and it is not necessarily due to a change to the query itself (or to the application code that calls it). Users often ask why a query suddenly got slower even though they haven’t published any changes to the application and the underlying data hasn’t changed drastically. This article points out some other reasons – and there are many – that a query might be slower today than it was 10 minutes ago, two weeks ago, or last summer.

Read on for a bulleted list of reasons. Of course, it would be extremely challenging to create a comprehensive list—for example, in the Same Plan section, in addition to there being more data, changes in the statistical distribution of data can cause performance profiles to change over time. But this is a really good starting point.