Why You Should Stop Looking At Query Cost

Spare No Expense


Over the years, there have been a lot of requests to get sp_BlitzCache to sort results by query cost. I understand why. It’s assumed that the optimizer is never wrong and that cost is directly associated with poor performance.

There are also rather misguided efforts to figure out parallelism settings based on plan costs. The main problem with that being that if you currently have a lot of parallel queries, all that means is that the estimated cost of the serial plan was higher than your current Cost Threshold For Parallelism setting, and the cost of the parallel plan was less than the cost of the serial plan.

If you increase Cost Threshold For Parallelism, you may very well still end up with a parallel plan, because the serial version was still more expensive. If you eventually change Cost Threshold For Parallelism to the point where some queries are no longer eligible for parallelism, you may eventually find yourself unhappy with the performance of the serial version of the query plan.

Albeit with less overall wait time on CX* doodads.

Next you’ll be complaining about all the SOS_SCHEDULER_YIELD waits you’ve got.

Insteads


Rather than look at estimated metrics, you should be looking at how queries actually perform. For most servers I look at, that means looking at queries with high average CPU time, and large memory grants. Those metrics typically represent tunable aspects of the query.

In other cases, you might look at wait stats to direct the type of queries you want to go after. Reads, writes, and executions are also valuable metrics at times.

One danger of looking at totals rather than averages is that you may find things that do a little bit of something a whole lot of times, and there’s no real way to tune the small bit of activity they generate other than to run the query less.

What’s A Cost For?


In general, I only tend to look at costs to figure out plan choices within a query, or when comparing two different plans for “the same” query.

This is where experimenting with hints to change the plan shapes and choices can show you why you got the plan you did, and what you might have to do to get the plan you want naturally.

Let’s say you want to figure out why you got a specific join type. You hint the type of join you want, and there’s a missing index request now. Adding the index gets you the plan shape you want without the hint. Everyone lived happily ever after.

Until the index got fragmented ???

Thanks for reading!



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