TOP versus SET ROWCOUNT

Question

Is there a difference in performance between TOP and SET ROWCOUNT or do they just get executed in the same manner?

Answer 1

Yes, functionally they are the same thing. As far as I know there are no significant performance differences between the two.

Just one thing to note is that once you have set rowcount this will persist for the life of the connection so make sure you reset it to 0 once you are done with it.

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EDIT (post Martin's comment)

The scope of SET ROWCOUNT is for the current procedure only. This includes procedures called by the current procedure. It also includes dynamic SQL executed via EXEC or SP_EXECUTESQL since they are considered "child" scopes.

Notice that SET ROWCOUNT is in a BEGIN/END scope, but it extends beyond that.

create proc test1
as
begin
    begin
    set rowcount 100
    end
    exec ('select top 101 * from master..spt_values')
end
GO

exec test1
select top 102 * from master..spt_values

Result = 100 rows, then 102 rows

Answer 2

One more note about performance, according to BOL:

As a part of a SELECT statement, the query optimizer can consider the value of expression in the TOP or FETCH clauses during query optimization. Because SET ROWCOUNT is used outside a statement that executes a query, its value cannot be considered in a query plan.

Article on BOL

Meaning there might be actually performance difference in these.