How to improve performance of SQL query with parameters? - sql

Question

I'm using SQL Server 2005.
I have a problems with executing SQL statements like this
DECLARE #Param1 BIT
SET #Param1 = 1
SELECT
t1.Col1,
t1.Col2
FROM
Table1 t1
WHERE
#Param1=0 OR
(t1.Col2 in
(SELECT t2.Col4
FROM
Table2 t2
WHERE
t2.Col1 = t1.Col1 AND
t2.Col2 = 'AAA' AND
t2.t3 <> 0)
)
This query executes very long time.
But if I replace #Param1 with 1, than query execution time is ~2 seconds.
Any information how to resolve the problem would be greatly appreciated.

Answer 1

Well, the explanation seems simple enough. For your current condition, since #Param1=0 is false (you set the parameter to 1 previously), it needs to evaluate your second condition, wich has a subquery and might take a long time. If you change your first filter to #Param1=1, then you are saying that it is true and there is no need to evaluate your second filter, hence making your query faster.

Answer 2

You seem to be confusing the optimiser with your OR statement. If you remove it, you should find it generates two different execution plans for the SELECT statement - one with a filter, and the other without:
DECLARE #Param1 BIT
SET #Param1 = 1
if #Param1=0
begin
SELECT
t1.Col1,
t1.Col2
FROM
Table1 t1
end
else
begin
SELECT
t1.Col1,
t1.Col2
FROM
Table1 t1
WHERE
(t1.Col2 in
(SELECT t2.Col4
FROM
Table2 t2
WHERE
t2.Col1 = t1.Col1 AND
t2.Col2 = 'AAA' AND
t2.t3 <> 0)
)
end

Answer 3

This is commonly referred to as the N+1 problem. You're doing a select in table 1 and for each record you find you will go look for something in table 2.
By setting your #Param1 to an value which will never be found in your select the sql engine will skip the subquery.
To avoid this behavior you could use a JOIN statement to join both tables together and afterwards filter the results with a where statement. The join statement will be a bit slower then a single subquery because you are matching 2 tables to eachother but because you only need to do the join once(vs N times) you'll gain a serious performance boost.
Example code :
DECLARE #Param1 BIT
SET #Param1 = 1
SELECT t1.Col1,t1.Col2
FROM Table1 t1
INNER JOIN Table2 t2 on t1.Col1 = t2.Col1
WHERE #Param1=0
OR t2.Col2 = 'AAA'
AND t2.t3 <> 0