Differences between xp_instance_RegRead and xp_RegRead

Question

Whats the difference between XP_REGREAD and XP_INSTANCE_REGREAD. How to use them, is there any docs on these?

Answer 1

xp_regread reads the literal registry path that you specify. xp_instance_regread "converts" the path you specify so that it matches the instance of SQL Server that you're currently using.

As an example, I have Developer and Express editions of SQL Server installed on my machine. I connect to the Express instance and run these two commands:

declare @dir nvarchar(4000) 

exec master.dbo.xp_regread 
    N'HKEY_LOCAL_MACHINE',
    N'Software\Microsoft\MSSQLServer\Setup',
    N'SQLPath',
    @dir output
select @dir

exec master.dbo.xp_instance_regread
    N'HKEY_LOCAL_MACHINE',
    N'Software\Microsoft\MSSQLServer\Setup',
    N'SQLPath', 
    @dir output
select @dir

The first returns C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL which is the value found at the location I specified but it is not the correct installation path for my Express instance.

The second returns c:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL which is the correct path for my Express instance. This value was actually read from HKEY_LOCAL_MACHINE\Software\Microsoft\Microsoft SQL Server\MSSQL.1\Setup which is quite different from the path I supplied to xp_instance_regread.