Re: - Mailing list pgsql-performance

Thanks Stephen,

My main concern is to get as much read performance on the disks as possible
on this given system.  CPU is rarely a problem on a typical data warehouse
system, this one's not any different.

We basically have 2 RAID5 disk sets (300Gb) and 150Gb) with a third one
coming along.(around 350Gb)
I was kind of hoping that the new PGSQL tablespaces would allow me to create
a storage container spanning multiple file-systems, but unfortunately, that
seems to be not the case.  Is this correct?

That tells me that I probably need to do a full reconfiguration of the disks
on the Solaris level to get maximum performance out of the system.
Mmmm. This is going to be a though one to crack.  Perhaps it will be
possible to get some extra juice out of placing the indexes on the smaller
disks (150G) and the data on the bigger ones?

Thanks!

Matt

-----Oorspronkelijk bericht-----
Van: Stephen Frost [mailto:sfrost@snowman.net]
Verzonden: donderdag 20 januari 2005 15:26
Aan: Matt Casters
CC: pgsql-performance@postgresql.org
Onderwerp: Re: [PERFORM]

* Matt Casters (Matt.Casters@advalvas.be) wrote:
> I have the go ahead of a customer to do some testing on Postgresql in
> a couple of weeks as a replacement for Oracle.
> The reason for the test is that the number of users of the warehouse
> is going to increase and this will have a serious impact on licencing
> costs. (I bet that sounds familiar)

Rather familiar, yes... :)

> We're running a medium sized data warehouse on a Solaris box (4CPU, 8Gb
RAM) on Oracle.
> Basically we have 2 large fact tables to deal with: one going for 400M
> rows, the other will be hitting 1B rows soon.
> (around 250Gb of data)

Quite a bit of data.  There's one big thing to note here I think- Postgres
will not take advantage of multiple CPUs for a given query, Oracle will.
So, it depends on your workload as to how that may impact you.  Situations
where this will be unlikely to affect you:

Your main bottle-neck is IO/disk and not CPU.
You run multiple queries in parallel frequently.
There are other processes on the system which chew up CPU time anyway.

Situations where you're likely to be affected would be:

You periodically run one big query.
You run a set of queries in sequential order.

> My questions to the list are: has this sort of thing been attempted
> before? If so, what where the performance results compared to Oracle?

I'm pretty sure it's been attempted before but unfortunately I don't have
any numbers on it myself.  My data sets aren't that large (couple million
rows) but I've found PostgreSQL at least as fast as Oracle for what we do,
and much easier to work with.

> I've been reading up on partitioned tabes on pgsql, will the
> performance benefit will be comparable to Oracle partitioned tables?

In this case I would think so, except that PostgreSQL still won't use
multiple CPUs for a given query, even against partitioned tables, aiui.

> What are the gotchas?

See above? :)  Other issues are things having to do w/ your specific
SQL- Oracle's old join syntax isn't supported by PostgreSQL (what is it,
something like select x,y from a,b where x=%y; to do a right-join, iirc).

> Should I be testing on 8 or the 7 version?

Now that 8.0 is out I'd say probably test with that and just watch for 8.0.x
releases before you go production, if you have time before you have to go
into production with the new solution (sounds like you do- changing
databases takes time anyway).

> Thanks in advance for any help you may have, I'll do my best to keep
> pgsql-performance up to date on the results.

Hope that helps.  Others on here will correct me if I misspoke. :)

    Stephen