I have a table with a xml column named Data:
CREATE TABLE [dbo].[Users](
[UserId] [int] IDENTITY(1,1) NOT NULL,
[FirstName] [nvarchar](max) NOT NULL,
[LastName] [nvarchar](max) NOT NULL,
[Email] [nvarchar](250) NOT NULL,
[Password] [nvarchar](max) NULL,
[UserName] [nvarchar](250) NOT NULL,
[LanguageId] [int] NOT NULL,
[Data] [xml] NULL,
[IsDeleted] [bit] NOT NULL,...
In the Data column there's this xml
<data>
<RRN>...</RRN>
<DateOfBirth>...</DateOfBirth>
<Gender>...</Gender>
</data>
Now, executing this query:
SELECT UserId FROM Users
WHERE data.value('(/data/RRN)[1]', 'nvarchar(max)') = @RRN
after clearing the cache takes (if I execute it a couple of times after each other) 910, 739, 630, 635, ... ms.
Now, a db specialist told me that adding a function, a view and changing the query would make it much more faster to search a user with a given RRN. But, instead, these are the results when I execute with the changes from the db specialist: 2584, 2342, 2322, 2383, ...
This is the added function:
CREATE FUNCTION dbo.fn_Users_RRN(@data xml)
RETURNS varchar(100)
WITH SCHEMABINDING
AS
BEGIN
RETURN @data.value('(/data/RRN)[1]', 'varchar(max)');
END;
The added view:
CREATE VIEW vwi_Users
WITH SCHEMABINDING
AS
SELECT UserId, dbo.fn_Users_RRN(Data) AS RRN from dbo.Users
Indexes:
CREATE UNIQUE CLUSTERED INDEX cx_vwi_Users ON vwi_Users(UserId)
CREATE NONCLUSTERED INDEX cx_vwi_Users__RRN ON vwi_Users(RRN)
And then the changed query:
SELECT UserId FROM Users
WHERE dbo.fn_Users_RRN(Data) = '59021626919-61861855-S_FA1E11'
Why is the solution with a function and a view going slower?
