Queries With Poor Cardinality Estimated or Skewed Parallelism

Dun Goofed

If you’re here, you hit an issue where SQL Server made a bad guess at how many rows it’d have to deal with in a query. This doesn’t always indicate a killer performance issue, but it does warrant investigation.

This can happen for a lot of reasons, including parameter sniffing, implicit conversion, non-SARGable predicates, functions, table variables, or just overly complicated queries.

Of course, it can also happen even if you do everything right.

Where To Look Next

You may not get any clues from the estimated/cached query plans, or the queries themselves. It could very well take running the query with the correct set of parameters to recreate and fix the issue.

You can get a lot of this information from sp_BlitzCache.