Subject: Re: Overhauling GUCS
From: Simon Riggs <simon@2ndquadrant.com>
To: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-hackers@postgresql.org
In-Reply-To: <48409D1E.3070208@agliodbs.com>
References: <48409D1E.3070208@agliodbs.com>
Content-Type: text/plain
Date: Sat, 31 May 2008 08:34:34 +0100
Message-Id: <1212219274.4120.191.camel@ebony.site>
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit


On Fri, 2008-05-30 at 17:34 -0700, Josh Berkus wrote:

> There's obviously some refinements needed, but what do people think of 
> the above general idea?

My initial reaction is that this is too much change, though I agree with
many of the points and understand it is well intended.

We have many supporters, though 90% of them seldom touch the database
from one release to the next. Many are dismayed that every time they do
we've fiddled with the knobs so some don't work anymore, some are
missing or renamed and there's a few new ones. So if they don't know
what they're doing it is frequently because we keep moving the
goalposts. 

We should also consider that most people with multiple databases are
running multiple versions of PostgreSQL. The main reason for that is
that we keep changing the behaviour of SQL between releases, so even if
you had a magic superfast upgrade utility, in perhaps 50% of cases they
won't use it because they have to do a full application re-test, which
costs time and money. 

Trying to be a Postgres DBA is hard when each new release is configured
differently to the last one and we have a major release about 3-5 more
frequently than Oracle/SQLServer. That is probably largest source of
questions and annoyance from the students on the courses I run.

So my viewpoint is that we should be aggressively adding new features,
yet be conservative in changing existing behaviour: provide options for
behaves-like-previous-release and keep the administrative interfaces as
similar as possible between releases.

If you wish to make changes, I would suggest that you simply reorder
some of the parameters in the .conf file, so that the ones you think are
important are grouped at the top.

Another suggestion would be to allow a #include style interface within
postgresql.conf. We can then do this:

#include standard_postgresql.conf

# now we put Josh's favourite GUCs as overrides on the above 
...

That keeps things simple because the standard_postgresql.conf looks
almost identical to what people are used to. It also provides a new
feature, allowing people to include site-wide defaults for various
settings to allow easy implementation of local policy. It also
demonstrates a best practice approach to managing GUCs.

Some other problems I see with GUCs

* It's not possible to set one parameter depending upon the setting of
another.

* It's always unclear which GUCs can be changed, and when. That is much
more infrequently understood than the meaning of them.

* We should rename effective_cache_size to something that doesn't sound
like it does what shared_buffers does

* There is no config verification utility, so if you make a change and
then try to restart and it won't, you are in trouble.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support