sql like query slow if using declare parameter but fast if not

Question

SQL 2008: This is slow (takes 1 1/2 minutes):

declare @p1 varchar(50)
set @p1 = '976j%'
select * from invsearch_query where comparepnfwd like @p1

This takes less than a second:

select * from invsearch_query where comparepnfwd like '976j%'

Why???

Answer 1

I would imagine that you must have a non covering index with leading column comparepnfwd that is used by the literal query but not by the query with the variable.

You can use OPTION (RECOMPILE) to get SQL Server to recompile the plan taking into account the actual variable value.