Re: [HACKERS] Slow count(*) again...

Mladen Gogala wrote:
> With all due respect, I don't see how does the issue of hints fall
> into this category? As I explained, the mechanisms are already there,
> they're just not elegant enough.

You're making some assumptions about what a more elegant mechanism would
look to develop that are simplifying the actual situation here. If you
take a survey of everyone who ever works on this area of the code, and
responses to this thread are already approaching a significant
percentage of such people, you'll discover that doing what you want is
more difficult--and very much "not elegant enough" from the perspective
of the code involved--than you think it would be.

It's actually kind of funny...I've run into more than one person who
charged into the PostgreSQL source code with the goal of "I'm going to
add good hinting!" But it seems like the minute anyone gets enough
understanding of how it fits together to actually do that, they realize
there are just plain better things to be done in there instead. I used
to be in the same situation you're in--thinking that all it would take
is a better UI for tweaking the existing parameters. But now that I've
actually done such tweaking for long enough to get a feel for what's
really wrong with the underlying assumptions, I can name 3 better uses
of development resources that I'd rather work on instead. I mentioned
incorporating cache visibility already, Robert has talked about
improvements to the sensitivity estimates, and the third one is
improving pooling of work_mem so individual clients can get more of it
safely.

> Well, those two databases are also used much more widely than
> Postgres, which means that they're doing something better than Postgres.

"Starting earlier" is the only "better" here. Obviously Oracle got a
much earlier start than either open-source database. The real
divergence in MySQL adoption relative to PostgreSQL was when they
released a Windows port in January of 1998. PostgreSQL didn't really
match that with a fully native port until January of 2005.

Check out
http://www.indeed.com/jobtrends?q=postgres%2C+mysql%2C+oracle&relative=1&relative=1
if you want to see the real story here. Oracle has a large installed
base, but it's considered a troublesome legacy product being replaced
whenever possible now in every place I visit. Obviously my view of the
world as seen through my client feedback is skewed a bit toward
PostgreSQL adoption. But you would be hard pressed to support any view
that suggests Oracle usage is anything other than flat or decreasing at
this point. When usage of one product is growing at an expontential
rate and the other is not growing at all, eventually the market share
curves always cross too.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books