Re: [HACKERS] Bad n_distinct estimation; hacks suggested?

Simon Riggs <simon@2ndquadrant.com> writes:
> My suggested hack for PostgreSQL is to have an option to *not* sample,
> just to scan the whole table and find n_distinct accurately.
> ...
> What price a single scan of a table, however large, when incorrect
> statistics could force scans and sorts to occur when they aren't
> actually needed ?

It's not just the scan --- you also have to sort, or something like
that, if you want to count distinct values.  I doubt anyone is really
going to consider this a feasible answer for large tables.

            regards, tom lane