SQL Server 查询：字面量快，变量慢

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, eg

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.

Hope this may help someone with similar issue.

Answer 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.

Answer 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.

Answer 6

Came across this same issue myself and it turned out to be a missing index involving a (left) join on the result of a subquery.

select *
from foo A
left outer join (
  select x, count(*)
  from bar
  group by x
) B on A.x = B.x

Added an index named bar_x for bar.x

Answer 7

I know this is long since answered, but I came across this same issue and have a fairly simple solution that doesn't require hints, statistics-updates, additional indexes, forcing plans etc.

Based on the comment above that "the optimizer cannot know that the value is not null", I decided to move the values from a variable into a table:

Original Code:

declare @StartTime datetime2(0) = '10/23/2020 00:00:00'
declare @EndTime datetime2(0) = '10/23/2020 01:00:00'
    
SELECT * FROM ...
WHERE 
C.CreateDtTm >= @StartTime
AND  C.CreateDtTm < @EndTime

New Code:

declare @StartTime datetime2(0) = '10/23/2020 00:00:00'
declare @EndTime datetime2(0) = '10/23/2020 01:00:00'

CREATE TABLE #Times (StartTime datetime2(0) NOT NULL, EndTime datetime2(0) NOT NULL)
INSERT INTO #Times(StartTime, EndTime) VALUES(@StartTime, @EndTime)

SELECT * FROM ...
WHERE 
C.CreateDtTm >= (SELECT MAX(StartTime) FROM #Times)
AND  C.CreateDtTm < (SELECT MAX(EndTime) FROM #Times)

This performed instantly as opposed to several minutes for the original code (obviously your results may vary) .

I assume if I changed my data type in my main table to be NOT NULL, it would work as well, but I was not able to test this at this time due to system constraints.

Answer 8

DECLARE @id INT = 1

SELECT * FROM View1 WHERE ID = @id

Do this

DECLARE @sql varchar(max)

SET @sql = 'SELECT * FROM View1 WHERE ID =' + CAST(@id as varchar)

EXEC (@sql)

Solves your problem