Bob Probst Bob Probst - 1 year ago 81
SQL Question

Rule of thumb on when to use WITH RECOMPILE option

I understand that the WITH RECOMPILE option forces the optimizer to rebuild the query plan for stored procs but when would you want that to happen?

What are some rules of thumb on when to use the WITH RECOMPILE option and when not to?

What's the effective overhead associated with just putting it on every sproc?

Answer Source

As others have said, you don't want to simply include WITH RECOMPILE in every stored proc as a matter of habit. By doing so, you'd be eliminating one of the primary benefits of stored procedures: the fact that it saves the query plan.

Why is that potentially a big deal? Computing a query plan is a lot more intensive than compiling regular procedural code. Because the syntax of a SQL statement only specifies what you want, and not (generally) how to get it, that allows the database a wide degree of flexibility when creating the physical plan (that is, the step-by-step instructions to actually gather and modify data). There are lots of "tricks" the database query pre-processor can do and choices it can make - what order to join the tables, which indexes to use, whether to apply WHERE clauses before or after joins, etc.

For a simple SELECT statement, it might not make a difference, but for any non-trivial query, the database is going to spend some serious time (measured in milliseconds, as opposed to the usual microseconds) to come up with an optimal plan. For really complex queries, it can't even guarantee an optimal plan, it has to just use heuristics to come up with a pretty good plan. So by forcing it to recompile every time, you're telling it that it has to go through that process over and over again, even if the plan it got before was perfectly good.

Depending on the vendor, there should be automatic triggers for recompiling query plans - for example, if the statistics on a table change significantly (like, the histogram of values in a certain column starts out evenly distributed by over time becomes highly skewed), then the DB should notice that and recompile the plan. But generally speaking, the implementers of a database are going to be smarter about that on the whole than you are.

As with anything performance related, don't take shots in the dark; figure out where the bottlenecks are that are costing 90% of your performance, and solve them first.

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