Don’t Be Afraid Of tempdb In SQL Server

It’s There For A Reason

When tuning queries, one pattern I see over and over again is people running crazy-long queries. Maybe they worked well-enough at some point, but over the years they just kept getting slower and slower.

Sometimes there are comments, and other times there’s enough domain knowledge on the call to understand how a query ended up in the shape it’s in. One persistent idea is that tempdb is something to be avoided. Either because it was “slow” or to avoid contention.

Granted, if a query has been around long enough, these may have been valid concerns at some point. In general though, temp tables (the # kind, not the @ kind) can be quite useful when query tuning.

You Might Be Using It, Anyway

Even if there’s some rule against directly using temp tables, queries can end up using tempdb by the caseload anyway.

Consider that Spool operators explicitly execute in tempdb, any spills will go to tempdb, and work tables that are used in a number of circumstances occur in tempdb. The bigger and more complicated your queries are, the more likely you are to run into cases where the optimizer Spools, Spills, or use some other workspace area in tempdb in your query plan.

Worse, optimizations available for temp tables aren’t available to on-the-fly operators. You also lose the ability to take further action by indexing your temp tables, etc.

It’s Often Easier Than Other Options

Many times when tuning queries, I’ll be puzzled by the optimizer’s choices. Sometimes it’s join type, other times it’s join order, or something else. Perhaps the most common reason is some misestimation, of course.

Query and index hints are great to experiment with, but are often unsatisfying as permanent fixes. I’m not saying to never use them, but you should explore other options first. In other words, keep temp tables on the table.

Thanks for reading!

Going Further

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