Question

Sql server 2008 R2

I have two queries

DECLARE @title NVARCHAR(500) = '"Finite" AND "Elements"'

select * from papers p
where (@title = '""' OR CONTAINS(p.name, @title))

select * from papers p
where (CONTAINS(p.name, @title))

first works about 7 seconds, second several miliseconds. WHY????

Answer 1

The @title = '""' OR ... bit is the problem.

At compile time it does not know whether @title is in fact going to contain the value "" and thus need to return all rows or not.

You could try adding OPTION(RECOMPILE) to the query so it is recompiled after the variable is assigned or just splitting it up into two cases.

IF @title = '""'
  SELECT *
  FROM   papers
ELSE
  SELECT *
  FROM   papers p
  WHERE  ( CONTAINS(p.name, @title) )