Parameter Sniffing Issues With Table Variables

Milos Radivojevic points out the downside to table variable deferred compilation in SQL Server 2019:

Since the actual number of rows is significantly greater than in the call with the parameter ‘White’, you can see here sort warnings because this time 1MB of memory grant was not sufficient for sorting. But, the execution plan is exactly the same as for the first call.

Prior to SQL Server 2019, the execution plan for the second query in this stored procedure was always the same, regardless of parameter used for the first invocation and thus plan generation. Since the table variable has cardinality of 1, all estimations and the execution plan will be the same. We can say, using a table variable in this stored procedure and passing it to the second query neutralizes parameter sniffing effect. That does not mean, this is good or bad for all executions (you saw sort warnings and they are always bad), but the plan was stable, it did not change even after failover or clearing cache. If you call this stored procedure usually with high selective parameters, you can consider this plan as a good plan.

In SQL Server 2019, since table variable could have different cardinality, this stored procedure is prone to parameter sniffing and depending on the first execution parameter, you can have different execution plans:

This is a natural outcome and something we’d work with just like we would with a temp table or regular table in a stored procedure.

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