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Another Batch of Common Query Plan Patterns

Erik Darling continues pulling one-star query plan patterns. First up is common subexpression spools:

The first time I heard the term “common subexpression spool” my brain went numb for a week.

It’s not a particularly mellifluous phrase, but it is helpful to understand what it is.

One easy way to think about it is a temporary cache for the result of a query, like a temp table inside your execution plan:

Then we look at optimizer choices around sorting:

Sometimes these Sorts are harmless, and sometimes they’re not. There are many situational things about the queries, indexes, available resources, and query plan appropriateness that will lead you to treating things differently.

Parameter sniffing, cardinality estimate accuracy, query concurrency, and physical memory are all potential reasons for these choices going great or going grog.

Read both of the whole things.