Sql query slow when using local variable

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I Have a problem with my query ,

It was fast when i using constant data but slow when i use a variable

example

Select * from T_report where convert(date,created_date) >='2020-05-01'
and convert(date,created_date)<='2020-05-30'

this query run fast ,

but when i use like this

declare @startdate as date
declare @enddate as date

set @startdate ='2020-05-01'
set @enddate='2020-05-30'

Select * from T_report where convert(date,created_date) >=@startdate
and convert(date,created_date)<=@enddate

What I have tried:

how i can solve this?

I have try using option(recompile)

but still slow

Posted 10-Jun-20 15:11pm

Member 14760154

Updated 11-Jun-20 2:23am

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Patrice T 10-Jun-20 22:31pm

Show table structure.

2 solutions

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Patrice T · Accepted Answer · 2020-06-10T19:04:00

SQL

Select * from T_report where convert(date,created_date) >='2020-05-01'
and convert(date,created_date)<='2020-05-30'

The problem I see is that conversion is done on wrong side of conditions.
Conversion must be done on constant, not on datafield (created_date).
The difference is that SQL compiler can fully optimize query when datafield is unchanged in condition.

CHill60 · Accepted Answer · 2020-06-11T02:23:00

In addition to Solution 1 - why are you converting column created_date - that column should be of type Date or DateTime

https://www.sqlshack.com/implementing-sql-data-types/[^]:
. . . always use the right data type for the job. . . It can have a significant impact on efficiency, performance, storage and further database development.

Change your table schema to make create_date a Date and your query becomes

SQL

Select * from T_report where date >= @startdate and created_date <= @enddate

Taking the convert function calls out of the WHERE clause will make a big difference to the speed.
If you already have created_date declared as a DateTime then instead of casting/converting the column in the WHERE clause, get clever with your local variables i.e.

SQL

set @startdate ='2020-05-01'
set @enddate='2020-05-31' -- Note this is the day after the end date!

Select * from T_report where created_date >=@startdate
and created_date<@enddate  -- Note only less than not less than or equal

One last final note - it's good practice to list the columns you want in your query rather than using * - it stops you getting caught out if the schema changes down the line

Sql query slow when using local variable

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