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.

Solution 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.

Solution 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))

Solution 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.

Solution 4

I ran into this problem myself with a view that ran < 10ms with a direct assignment (WHERE UtilAcctId=12345), but took over 100 times as long with a variable assignment (WHERE UtilAcctId = @UtilAcctId).
The execution-plan for the latter was no different than if I had run the view on the entire table.

My solution didn't require tons of indexes, optimizer-hints, or a long-statistics-update.

Instead I converted the view into a User-Table-Function where the parameter was the value needed on the WHERE clause. In fact this WHERE clause was nested 3 queries deep and it still worked and it was back to the < 10ms speed.

Eventually I changed the parameter to be a TYPE that is a table of UtilAcctIds (int). Then I can limit the WHERE clause to a list from the table. WHERE UtilAcctId = [parameter-List].UtilAcctId. This works even better. I think the user-table-functions are pre-compiled.

Solution 5

When SQL starts to optimize the query plan for the query with the variable it will match the available index against the column. In this case there was an index so SQL figured it would just scan the index looking for the value. When SQL made the plan for the query with the column and a literal value it could look at the statistics and the value to decide if it should scan the index or if a seek would be correct.

Using the optimize hint and a value tells SQL that “this is the value which will be used most of the time so optimize for this value” and a plan is stored as if this literal value was used. Using the optimize hint and the sub-hint of UNKNOWN tells SQL you do not know what the value will be, so SQL looks at the statistics for the column and decides what, seek or scan, will be best and makes the plan accordingly.

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Comments

[Admin]

+1 Works like a champ here (and much less manual than trying to force various joins/plans, etc.)

[Martin Smith]

@pst - This is the right suggested solution but wrong reasoning. The OP has a variable not a parameter. And the issue is selectivity estimates as SQL Server doesn't do variable sniffing. Not "creating a plan that deals with NULL"

[Admin]

@MartinSmith All the little details that escape me :( All I know is that SQL Server was happily turning a 2 second (in SMSS query) into a I'm-gonna-stop-it-after-10-minute query an SqlCommand (with placeholders) as part of a Typed DataSet. I updated all the statistics (without any options) on the related tables before trying the OPTION(RECOMPILE) route, but that didn't seem to fix the issue. What is meant by the "creating a plan that deals with NULL"?

[Martin Smith]

@pst - I was referring to the phrase in this answer " 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.". Maybe the OP was thinking of the pattern WHERE ID = @id or @id IS NULL sometimes used for optional parameters when they wrote it.

[Admin]

@MartinSmith Ahh. I just skipped to the ATTENTION(GRABBING) suggestion -- in my case the +1 was for a "my problem is fixed" :-)

[John C]

The suggestion here regarding the optimiser allowing for the possibility of nulls helped me; I wrapped my variable with a COALESCE statement and the SP execution sped right up. I should mention I was generating dynamic SQL and was only adding that element of the where clause if the variable was not null.

[quinz]

Please elaborate what your snippet does and why it solves the problem.

[Yahya Hussein]

Please add some explanation

[user2864740]

Hah, I wrote this question 8 years ago and I found it again.. for the same issue a few SQL Server versions later. SQL Server is not correctly promoting the filter into the nested table queries in my view when a parameter is used - a SPROC (or TVF) with this promotion done manually results in expected fast performance.