Re: LIMIT confuses the planner

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



> So the bottom line here is just that the estimated n_distinct is too
> low.  We've seen before that the equation we use tends to do that more
> often than not.  I doubt that consistently erring on the high side would
> be better though :-(.  Estimating n_distinct from a limited sample of
> the population is known to be a statistically hard problem, so we'll
> probably not ever have perfect answers, but doing better is on the
> to-do list.
>

I hit an interestinhg paper on n_distinct calculation:

http://www.pittsburgh.intel-research.net/people/gibbons/papers/distinct-values-chapter.pdf

the PCSA algorithm described there requires O(1) calculation per
value. Page 22 describes what to do with updates streams.

This I think (disclaimer: I know little about PG internals) means that
the n_distinct estimation can be done during vacuum time (it would
play well with the visibility map addon).

What do You think?

Greetings
Marcin

-- 
Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance


[Postgresql General]     [Postgresql PHP]     [PHP Users]     [PHP Home]     [PHP on Windows]     [Kernel Newbies]     [PHP Classes]     [PHP Books]     [PHP Databases]     [Yosemite]

  Powered by Linux