Parameter sniffing on table valued parameters

Question

I'm fairly certain that adding parameter sniffing to table valued parameters is of little or no value however I was wondering if someone could confirm this?

(INT_LIST is a user defined table type which is a single column of type INT)

CREATE PROCEDURE [dbo].[TVPSniffTest](
    @param1 varchar(50),
    @idList INT_LIST readonly
)
AS
BEGIN 
   DECLARE @param1_sniff VARCHAR(50) = @param1 --this is worth doing

   DECLARE @idList_sniff INT_LIST
   INSERT INTO @idList_sniff SELECT value FROM @idList --will this help?

   --query code here
END

Answer 1

This has no effect whatsoever -- in fact, it's detrimental to performance because you're copying the whole table first.

The optimizer maintains no statistics for either table-valued parameters or table variables. This can easily lead to bad query plans with cardinality mismatches; the solution for that is usually an intermediate temp table. In any case, parameter sniffing won't be an issue -- the table contents are never used to optimize the query plan.

Incidentally, while you can assign the parameter to a local variable to circumvent sniffing, a more flexible option is to use the OPTIMIZE FOR or RECOMPILE hints in queries that are particularly affected (or WITH RECOMPILE on the whole stored procedure, but that's a little more drastic). This prevents cluttering the procedure with copies of everything.