Question

Is there a performance difference between the following 2 queries, and if so, then which one is better?:

    select 
    q.id, 
    q.name 
    from(
        select id, name, row_number over (partition by name order by id desc) as row_num
from table
    ) q
        where q.row_num = 1

versus

select
max(id) ,
name
from table
group by name

(The result set should be the same)

This is assuming that no indexes are set.

UPDATE: I tested this, and the group by was faster.

Answer 1

The group by should be faster. The row number has to assign a row to all rows in the table. It does this before filtering out the ones it doesn’t want.

The second query is, by far, the better construct. In the first, you have to be sure that the columns in the partition clause match the columns that you want. More importantly, “group by” is a well-understood construct in SQL. I would also speculate that the group by might make better use of indexes, but that is speculation.