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Trying out Parameter Sensitive Plan Optimization

Jared Poche gives it a try:

Parameter Sensitive Plan Optimization attempts to solve the parameter sniffing issue by allowing a query to have different plans that are used based on the cardinality of one important parameter. The parent query has a plan that is really just a stub; there can be up to three variant queries that have full execution plans. When the query is executed, the optimizer chooses which variant query and plan to use based on the cardinality of the parameter’s value.

Click through for examples. I share Jared’s thoughts about row counts, though I’m not a huge fan of just using those. The rule of thumb I like to apply (though I certainly didn’t make it up myself!) is that, if you’re retrieving 0.5% or less of a table, a seek is the best option, assuming there is an appropriate index. If you’re retrieving 20% or more of a table, a scan is the best option, regardless of index quality.

Going a little further, somewhere in that 0.5-20% range, there’s an in-between zone where you should be indifferent between seek and scan, as they’ll both perform approximately as well. But if the optimizer chooses “seek” for the in-between zone and you nudge up those returned row counts higher and higher, seek becomes less viable, and there may be a zone somewhere between X% (that mid-point of indifference) and 20% where you haven’t yet crossed the row count threshold for another plan but should switch over to the scan.

Coming up with the right solution to this problem would be pretty hard, and I’m not paid to solve problems. I’m not-paid to come up with problems, however.