> 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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