The spool operator is one of those icons that will show up in a query plan now and then. In this post, we’ll take a closer look at what it does in a query plan, how it affects performance, and what you can do to substitute or eliminate it.
Knowing how to read a query plan is absolutely key to optimizing SQL Server query performance. The query plan tells you how SQL Server goes about running your query, including what indexes are used (and how), what join strategies are applied and a lot of other information. If you can read the query plan, you can make the appropriate changes to indexes, query hints, join conditions, etc to tune your workload for optimum performance.