Is my understanding of "select distinct" correct?
- by paxdiablo
We recently discovered a performance problem with one of our systems and I think I have the fix but I'm not certain my understanding is correct.
In simplest form, we have a table blah into which we accumulate various values based on a key field. The basic form is:
recdate date
rectime time
system varchar(20)
count integer
accum1 integer
accum2 integer
There are a lot more accumulators than that but they're all of the same form. The primary key is made up of recdate, rectime and system.
As values are collected to the table, the count for a given recdate/rectime/system is incremented and the values for that key are added to the accumulators. That means the averages can be obtained by using accumN / count.
Now we also have a view over that table specified as follows:
create view blah_v (
recdate, rectime, system, count,
accum1,
accum2
) as select distinct
recdate, rectime, system, count,
value (case when count > 0 then accum1 / count end, 0),
value (case when count > 0 then accum2 / count end, 0)
from blah;
In other words, the view gives us the average value of the accumulators rather than the sums. It also makes sure we don't get a divide-by-zero in those cases where the count is zero (these records do exist and we are not allowed to remove them so don't bother telling me they're rubbish - you're preaching to the choir).
We've noticed that the time difference between doing:
select distinct recdate from XX
varies greatly depending on whether we use the table or the view. I'm talking about the difference being 1 second for the table and 27 seconds for the view (with 100K rows).
We actually tracked it back to the select distinct. What seems to be happening is that the DBMS is actually loading all the rows in and sorting them so as to remove duplicates. That's fair enough, it's what we stupidly told it to do.
But I'm pretty sure the fact that the view includes every component of the primary key means that it's impossible to have duplicates anyway. We've validated the problem since, if we create another view without the distinct, it performs at the same speed as the underlying table.
I just wanted to confirm my understanding that a select distinct can not have duplicates if it includes all the primary key components. If that's so, then we can simply change the view appropriately.
