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

Simon, Tom:

While it's not possible to get accurate estimates from a fixed size sample, I
think it would be possible from a small but scalable sample: say, 0.1% of all
data pages on large tables, up to the limit of maintenance_work_mem.

Setting up these samples as a % of data pages, rather than a pure random sort,
makes this more feasable; for example, a 70GB table would only need to sample
about 9000 data pages (or 70MB). Of course, larger samples would lead to
better accuracy, and this could be set through a revised GUC (i.e.,
maximum_sample_size, minimum_sample_size).

I just need a little help doing the math ... please?

--
--Josh

Josh Berkus
Aglio Database Solutions
San Francisco