SQL Server Query Optimization: Where (Col=@Col or @Col=Null)

Question

Not sure where to start on this one -- not sure if the problem is that I\'m fooling the query optimizer, or if it\'s something intrinsic to the way indexes work when nulls a

Answer 1

You have a combination of issues, most likely

1. Parameter sniffing
2. OR is not a good operator to use

But without seeing the plans, these are educated guesses.

Parameter sniffing

... of the default "NULL". Try it with different defaults, say -1 or no default.

The @ID = -1 with a default of NULL and parameter sniffing = trivial check, so it's faster.

You could also try OPTIMISE FOR UNKNOWN in SQL Server 2008

The OR operator

Some ideas..

If the columns is not nullable, in most cases the optimiser ignores the condition

st.ID = ISNULL(@ID, st.ID)

Also, you can use IF statement

IF @ID IS NULL
   SELECT ... FROM...
ELSE
   SELECT ... FROM... WHERE st.ID

Or UNION ALL in a similar fashion.

Personally, I'd use parameter masking (always) and ISNULL in most cases (I'd try it first)

alter procedure SomeProc
  @ID int = NULL
AS
declare @maskID int
select @maskID = @ID
...