Re: WIP: cross column correlation ...

Robert Haas wrote:
> If you want to take the above as in any way an exhaustive survey of
> the landscape (which it isn't), C seems like a standout, maybe
> augmented by the making the planner able to notice that A1 = x1 AND A2
> = x2 is equivalent to (A1,A2) = (x1, x2) so you don't have to rewrite
> queries as much.
> 
> I don't really know how to handle the join selectivity problem.  I am
> not convinced that there is a better solution to that than decorating
> the query.  After all the join selectivity depends not only on the
> join clause itself, but also on what you've filtered out of each table
> in the meantime.

Thinking some more, I think another downside to the "decorate the query"
idea is that many queries use constants that are supplied only at
runtime, so there would be no way to hard-code a selectivity value into
a query when you don't know the value.  Could a selectivity function
handle that?

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +