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Re: is it possible to make this faster?

From: "Merlin Moncure" <mmoncure(at)gmail(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Postgresql Performance" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: is it possible to make this faster?
Date: 2006-05-26 17:46:30
Message-ID: b42b73150605261046w78e48f72j5f080db0e4ee5370@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On 5/26/06, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> "Merlin Moncure" <mmoncure(at)gmail(dot)com> writes:
> > did you have a key on a,b,c?
> Yeah, I did
>         create index t1i on t1 (a,b,c);
> Do I need to use some other syntax to get it to work?

can't thing of anything, I'm running completely stock, did you do a
optimize table foo? is the wind blowing in the right direction?

> > select count(*) from (select a,b,max(c) group by a,b) q;
> > blows the high performance case as does putting the query in a view.

> I noticed that too, while trying to suppress the returning of the
> results for timing purposes ... still a few bugs in their optimizer
> obviously.  (Curiously, EXPLAIN still claims that the index is being
> used.)

well, they do some tricky things pg can't do for architectural reasons
but the special case is obviously hard to get right.  I suppose this
kinda agrues against doing all kinds of acrobatics to optimize mvcc
weak cases like the above and count(*)...better to make heap access as
quick as possible.

> > mysql> select user_id, acc_id, max(sample_date) from usage_samples group by 1,2
> > 939 rows in set (0.07 sec)

> > mysql> select user_id, acc_id, max(sample_date) from usage_samples group by 1,2
> > 939 rows in set (1.39 sec)

oops, pasted the wrong query..case 2 should have been
select user_id, acc_id, max(sample_date), disksize from usage_samples
group by 1,2
illustrating what going to the heap does to the time.

merlin

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