Slow T-SQL query with datediff function - sql

Question

I have a query which runs fast when the date clause "and datediff(day,con2.DT_DateIncluded),'2017-01-01')<=0" in the code below isn't used in the query, but runs slowly when it is included. Though it runs fast when I run just the part "select top 2 ID_Contact...", even including the date clause. I have this query on a classic ASP application, and it can't be converted in to a stored procedure (project scope reasons). Can you help me find a way to improve the performance of the full query just by changing the query code?
select distinct top 10
ID_Contact, NO_CodCompany
from
tblContacts con1
where
ID_Contact in (select top 2 ID_Contact
from tblContacts con2
inner join tblCompanies cp on con2.NO_CodCompany = cp.ID_Company
where con2.NO_CodCompany = con1.NO_CodCompany
and datediff(day, con2.DT_DateIncluded), '2017-01-01') <= 0)

Answer 1

Instead of `DATEDIFF() < 0' try using:
and con2.DT_DateIncluded <= '2017-01-01'
Also, ensure that there is an index on the `DT_DateIncluded' column.
The reason DATEDIFF() runs slow is that using it takes a bit of time to perform the calculation, the query optimizer is (probably) ending up running it for the entire table, and there is (probably) no index to help it select the required rows.
When you remove that clause the query runs faster, but that is probably helped along by the fact that you're only selecting the first two rows in the inner query and ten rows in the outer query, allowing a table scan to be performant enough.

Answer 2

This is essentially your query:
This is your query:
select distinct top 10 ID_Contact, NO_CodCompany
from tblContacts con1
where ID_Contact in (select top 2 ID_Contact
from tblContacts con2 inner join
tblCompanies cp
on con2.NO_CodCompany = cp.ID_Company
where con2.NO_CodCompany = con1.NO_CodCompany and
datediff(day, con2.DT_DateIncluded), '2017-01-01') <= 0
);
My first suggestion is to change the datediff() to a simple date comparison:
select distinct top 10 ID_Contact, NO_CodCompany
from tblContacts con1
where ID_Contact in (select top 2 ID_Contact
from tblContacts con2 inner join
tblCompanies cp
on con2.NO_CodCompany = cp.ID_Company
where con2.NO_CodCompany = con1.NO_CodCompany and
con2.DT_DateIncluded < '2017-01-02'
);
Then, I would remove the JOIN in the subquery. I'm not 100% sure this is exactly equivalent, because that might depend on nuances in the data:
select distinct top 10 ID_Contact, NO_CodCompany
from tblContacts con1
where con1.ID_Contact in (select top 2 con2.ID_Contact
from tblCompanies cp
where con1.NO_CodCompany = cp.ID_Company and
con1.DT_DateIncluded < '2017-01-02'
);
Then, if you can remove the select distinct in the outermost query, you should do that.

Answer 3

Try this instead:
con2.DT_DateIncluded < '20170102'
It's better because it still allows the server to make use of any indexes on the DT_DateIncluded column. Currently, this is not possible. Even worse, the query is probably having to run that DATEDIFF() function on every record in the table.
Note that this is equivalent to what you posted, even if it might not match what you intended. I suspect con2.DT_DateIncluded < '20170101' is closer to what you really meant.
I also suspect you could do this either without the 2nd instance of tblContacts or with a windowing function to get much better results, or at least by using JOIN instead of IN to filter the results.
Finally, for historical reasons, when entering a date-only value, you should use the unseparated date format as described here:
The ultimate guide to the datetime datatypes
For date/time values, you can still use the separated yyyy-mm-dd hh:mm:ss you're used to, but if you only have the date part, yyyymmdd is better.
Based on this comment:
My goal with this query is to obtain contacts from companies but limited to "n" contacts per company
You should look into the APPLY operator. Unfortunately, it's still not clear to me how everything fits together, but I will least provide a demonstration using the APPLY operator to show two contacts per company that you can use as a starting point:
SELECT TOP 10 ct.ID_Contact, ct.NO_CodCompany
FROM tblCompanies cp
CROSS APPLY (
SELECT TOP 2 ID_Contact, NO_CodCompany
FROM tblContacs
WHERE NO_CodCompany = cp.ID_Company
AND DT_DateIncluded < '20170102'
ORDER BY DT_DateIncluded DESC
) ct
APPLY works kind of like a JOIN on a nested SELECT query, where there is no ON clause; the join conditional is instead included as part of the WHERE clause in the nested SELECT statement.
Note the use of CROSS. This will exclude companies that have no contacts at all. If you want to include those companies, change it to OUTER.
You should also look at what indexes you have defined. A single index on the tblContacts table that looks at NO_CodCompany and DT_DateIncluded (in that order!) might work wonders for this query, especially if it also has ID_Contact in the INCLUDES clause. Then you could complete the tblContacts portion of the query entirely from the index.