Re: The science of optimization in practical terms?

On Fri, Feb 13, 2009 at 3:27 PM, Joshua D. Drake <jd@commandprompt.com> wrote:
> On Fri, 2009-02-13 at 20:10 +0000, Grzegorz Jaskiewicz wrote:
>> yet more arguments, to let postgresql estimate those automatically.
>
> Well I haven't seen any arguments actually. Which was the point of my
> original question. I don't think anyone actually knows what these knobs
> change, in practice.

Well, in broad strokes, it seems to me that what they do is fairly
obvious: they affect the planner's willingness to choose plans that
touch more pages vs. plans that involve more CPU overhead (e.g. qual
evaluation).  If the database is mostly or entirely in shared buffers
or the system buffer cache, and CPU consumption is a problem, then
raising the CPU costs is apt to help.

I think the root of this problem is that we can't model caching
effects.  random_page_cost > seq_page_cost models the cost of seeks,
but min(random_page_cost, seq_page_cost) >> max(cpu_tuple_cost,
cpu_index_tuple_cost, cpu_operator_cost) models the fact that read
from disk, even sequentially, is always slow.  Unfortunately, if the
whole database is likely already in memory, which seems to be a pretty
common scenario even for relatively large databases (because people
buy more memory to make them fit), then it's just wrong.

If we had a good knowledge of which pages were apt to be cached, we
could add a GUC cached_page_cost with a default value of maybe 0.2,
and presumably we'd get better plans that way.  The bad news is that
it's pretty difficult to get that knowledge (and of course it could
change after the fact if the usage pattern of the database shifts
dramatically).  The good news is that experimentation is possible.
For example, we could:

- Assume that small relations are more likely to be cached (so derate
page costs when accessing them).
- Allow users to override the page cost on a per-rel basis using a reloption.
- Gather statistics on relation access patterns and use that to
estimate the fraction of a relation likely to be in cache.

If your whole database stays in memory all the time, I would guess
that you could either raise the CPU costs or drop the page costs quite
substantially and that would probably work out fine.  What's tougher
is to still be able to generate good plans when only part of the
database fits in memory, or there's other activity on the system that
is periodically purging portions of the system cache.

...Robert