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Re: More thoughts about planner's cost estimates

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: More thoughts about planner's cost estimates
Date: 2006-06-02 22:36:40
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
I wrote:
> In general it seems to me that for CPU-bound databases, the default values
> of the cpu_xxx_cost variables are too low.  ... rather than telling people
> to manipulate all three of these variables individually, I think it might
> also be a good idea to provide a new GUC variable named something like
> "cpu_speed_scale" that would just be a multiplier for the other variables.
> It would default to 1.0 and our standard advice for CPU-bound databases
> would be "decrease random_page_cost to 1.0 and raise cpu_speed_scale to
> 10.0 or so".  Seems cleaner than telling people to muck with three or so
> individual values.

Nicolai Petri's comment about per-tablespace access costs caused me to
rethink the above proposal.  Instead of inventing "cpu_speed_scale",
which seems rather baroque after thinking about it more, what I now
think we should do is invent a "seq_page_cost" GUC to replace the
traditionally hardwired value of 1.0 cost unit per sequential page
fetch.  Then, if you've got different tablespaces with different disk
speeds, you could imagine having per-tablespace values of seq_page_cost
and random_page_cost, whereas you probably want the CPU cost numbers
to remain the same across all tables.

I don't really want to get into inventing per-tablespace settings right
now, because the need hasn't been demonstrated; but if we ever do want
to do it, this approach will be a whole lot less confusing than
something involving a cpu_speed_scale knob.

This still leaves you twiddling two knobs (now random_page_cost and
seq_page_cost) if you want to set up the planner for an all-in-memory
database.  So it's not any more complicated for that purpose.

One objection to this is that after moving "off the gold standard" of
1.0 = one page fetch, there is no longer any clear meaning to the
cost estimate units; you're faced with the fact that they're just an
arbitrary scale.  I'm not sure that's such a bad thing, though.  For
instance, some people might want to try to tune their settings so that
the estimates are actually comparable to milliseconds of real time.


			regards, tom lane

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