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Re: Performance of count(*)

From: "Merlin Moncure" <mmoncure(at)gmail(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Performance of count(*)
Date: 2007-03-22 18:24:39
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On 3/22/07, Michael Stone <mstone+postgres(at)mathom(dot)us> wrote:
> On Thu, Mar 22, 2007 at 06:27:32PM +0100, Tino Wildenhain wrote:
> >Craig A. James schrieb:
> >>You guys can correct me if I'm wrong, but the key feature that's missing
> >>from Postgres's flexible indexing is the ability to maintain state
> >>across queries.  Something like this:
> >>
> >>  select a, b, my_index_state() from foo where ...
> >>    offset 100 limit 10 using my_index(prev_my_index_state);
> >>
> >
> >Yes, you are wrong :-) The technique is called "CURSOR"
> >if you maintain persistent connection per session
> >(e.g. stand allone application or clever pooling webapplication)
> Did you read the email before correcting it? From the part you trimmed
> out:
> >The problem is that relational databases were invented before the web
> >and its stateless applications.  In the "good old days", you could
> >connect to a database and work for hours, and in that environment
> >cursors and such work well -- the RDBMS maintains the internal state of
> >the indexing system.  But in a web environment, state information is
> >very difficult to maintain.  There are all sorts of systems that try
> >(Enterprise Java Beans, for example), but they're very complex.
> It sounds like they wrote their own middleware to handle the problem,
> which is basically what you suggested (a "clever pooling web
> application") after saying "wrong".

Tino was saying that rather that build a complete indexing storage
management solution that lives outside the database, it is better to
do intelligent session management so that you get the simplicity if a
two tier client server system but the scalability of a web app.

Web apps are not necessarily stateless, you just have to be a little
clever about how database connections are opened and closed.  Then you
get all the database stuff that comes along with a persistent
connection (advisory locks, cursors, prepared statements, etc) without
building all kinds of data management into the middleware.


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