Re: per table random-page-cost?

Le lundi 19 octobre 2009 23:14:40, Robert Haas a écrit :
> On Mon, Oct 19, 2009 at 5:08 PM, marcin mank <marcin(dot)mank(at)gmail(dot)com> wrote:
> > Currently random_page_cost is a GUC. I propose that this could be set
> > per-table.
> >
> > I think this is a good idea for widely-wanted planner hints. This way
> > You can say "I do NOT want this table to be index-scanned, because I
> > know it is not cached" by setting it`s random_page_cost to a large
> > value (an obviously You can do the other way around, when setting the
> > random_page_cost to 1 You say "I don`t care how You fetch the pages,
> > they are all in cache")
> >
> > The value for the per-table setting could be inferred from
> > pg_stat(io)?.*tables . We could have a tool to suggest appropriate
> > values.
> >
> > We could call it something like cached_percentage (and have the cost
> > of a random tuple fetch be inferred from the global random_page_cost,
> > seq_tuple_cost and the per-table cached_percentage). Then we could set
> > the global random_page_cost to a sane value like 200. Now one can
> > wonder why the planner works while having such blantantly unrealistic
> > values for random_page_cost :)
> >
> > What do You think?
>
> I've been thinking about this a bit, too. I've been wondering if it
> might make sense to have a "random_page_cost" and "seq_page_cost"
> setting for each TABLESPACE, to compensate for the fact that different
> media might be faster or slower, and a percent-cached setting for each
> table over top of that.

At least settings by TABLESPACE should exists. I totaly agree with that.

>
> ...Robert
>

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