Re: proposal : cross-column stats

On Fri, Dec 17, 2010 at 12:58 PM, Tomas Vondra <tv(at)fuzzy(dot)cz> wrote:
> In the end, all they need to compute an estimate is number of distinct
> values for each of the columns (we already have that in pg_stats) and a
> number of distinct values for the group of columns in a query. They
> really don't need any multidimensional histogram or something like that.

I haven't read the paper yet (sorry) but just off the top of my head,
one possible problem here is that our n_distinct estimates aren't
always very accurate, especially for large tables. As we've discussed
before, making them accurate requires sampling a significant
percentage of the table, whereas all of our other statistics can be
computed reasonably accurately by sampling a fixed amount of an
arbitrarily large table. So it's possible that relying more heavily
on n_distinct could turn out worse overall even if the algorithm is
better. Not sure if that's an issue here, just throwing it out
there...

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company