Code Optimization

Interesting things in software development and code optimization

SQL COUNT(*) is slow - how to increase the performance

Hi friends,

today I'm going to show you one possible way to speed up the COUNT(*)  performance in case when you need to know the total amount of rows of your select query with paging.

Here is the example of my first and slow query:

Select

    COUNT(*) OVER () as TotalCount

    , ID

    , OwnerID

    , Name

    , [Description]

    , keywords

From [dbo].[tblData]

where pState = N'a' and [status] <> 'P'

    and (@FilterBy = 0 OR @FilterBy = TypeID)

Order By DateAdded Desc

OFFSET @p0 ROWS FETCH NEXT @p1 ROWS ONLY;


this query did take from 4 seconds up to 9 seconds selecting 315000 rows from about 450000 rows -

that is really slow. It seems like OVER () overloads the query itself (but this is just my guess).

Also converting from nchar to char takes a lot of time, so I did change it (notice the N letter before string)

After some time of thinking and playing with that around, I did come to the following solution:

select @rowstotal = count(*)

From [dbo].[tblData]

where pState = 'a' and [status] <> 'P'

     and (@FilterBy = 0 OR @FilterBy = TypeID)


select

      @rowstotal as TotalCount

    , ID

    , OwnerID

    , Name

    , [Description]

    , keywords

From [dbo].[tblData]

where pState = 'a' and [status] <> 'P'

    and (@FilterBy = 0 OR @FilterBy = TypeID)

Order By DateAdded Desc

OFFSET @p0 ROWS FETCH NEXT @p1 ROWS ONLY;


(pay attention that to calculate total number of rows to be selected you do not need ordering. I think count(*) with ordering works much slower)


Now this query takes up to 1 second to select the same 315000 rows from about 450000 that is minimum 4 times faster :) 


I'm not sure if this is really best solution as I'm not a DBA master :)

but in my case it sped up the query enough.


Thank you.


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