SQL Server Query: Fast with Literal but Slow with Variable

Question

I have a view that returns 2 ints from a table using a CTE. If I query the view like this it runs in less than a second

SELECT * FROM view1 WHERE ID = 1

However if I query the view like this it takes 4 seconds.

DECLARE @id INT = 1
SELECT * FROM View1 WHERE ID = @id

I've checked the 2 query plans and the first query is performing a Clustered index seek on the main table returning 1 record then applying the rest of the view query to that result set, where as the second query is performing an index scan which is returning about 3000 records records rather than just the one I'm interested in and then later filtering the result set.

Is there anything obvious that I'm missing to try to get the second query to use the Index Seek rather than an index scan. I'm using SQL 2008 but anything I do needs to also run on SQL 2005. At first I thought it was some sort of parameter sniffing problem but I get the same results even if I clear the cache.

Answer 1

Probably it is because in the parameter case, the optimizer cannot know that the value is not null, so it needs to create a plan that returns correct results even when it is. If you have SQL Server 2008 SP1 you can try adding OPTION(RECOMPILE) to the query.

Answer 2

You could add an OPTIMIZE FOR hint to your query, e.g.

DECLARE @id INT = 1
SELECT * FROM View1 WHERE ID = @id OPTION (OPTIMIZE FOR (@ID = 1))

Answer 3

In my case in DB table column type was defined as VarChar and in parameterized query parameter type was defined as NVarChar, this introduced CONVERT_IMPLICIT in the actual execution plan to match data type before comparing and that was culprit for sow performance, 2 sec vs 11 sec. Just correcting parameter type made parameterized query as fast as non parameterized version.

One possible way to do that is to CAST the parameters, as such:

SELECT ...
FROM ...
WHERE name = CAST(:name AS varchar)

Hope this may help someone with similar issue.