Question

I’m writing a general search Stored Procedure to search in a table based on many filters which user can select in the UI (using MS-SQL 2008).

Here is ther simplified version:

CREATE PROCEDURE SearchAll
    @FirstName NVARCHAR(MAX) = NULL,
    @LastName NVARCHAR(MAX) = NULL,
    @Age INT = NULL
AS
    SELECT * 
    FROM persons 
    WHERE 
        (@FirstName IS NULL OR FirstName = @firstname)
        AND (@LastName IS NULL OR LastName = @LastName)
        AND (@Age IS NULL OR Age = @Age)

It seems that if I pass NULL to @Age there’ll be no performance cost. But, when I’m testing with huge amount of data, I have a great perfomance lost!

Here is the queries which are the same logicaly but VERY different practically:

DECLARE @FirstName NVARCHAR(MAX) = NULL
DECLARE @Age INT = 23
------------First slow------------
SELECT * 
FROM persons 
WHERE 
    (@FirstName IS NULL OR FirstName = @firstname)
    AND (@Age IS NULL OR Age = @Age)
------------Very fast------------
SELECT * 
FROM persons 
WHERE 
    Age = @Age

Did is miss a point?

I know that SQL engine finds the best match for indexes, and … (before running the query),but it’s obvious that: @FirstName IS NULL and there’s no need to analyse anything.

I’ve also tested ISNULL function in the query (the same result).

Answer 1

Queries that contain this construct of @variable is null or @variable = column are a performance disaster. This is because SQL plans are created so they work for any value of the variable. For a lengthy discussion of the topic, the problems and possible solutions see Dynamic Search Conditions in T-SQL