Re: Multi-column distinctness.

I am getting more attracted to your suggestion of making these things
stand-alone commands "CREATE STATISTICS" and "DROP STATISTICS".  Not only
does that fix the syntactic problems of shoehorning them into ALTER TABLE,
but it would be possible to design the syntax to allow for straightforward
extension to multiple-table cases.  Even if we don't know what we'd do
with that just yet, it seems to me like a possible future extension that
we should keep in mind.  And anything based on ALTER TABLE just cannot do
that reasonably.

So consider

CREATE STATISTICS name ON table(columnlist) [, table(columnlist) ...]
[ WITH options ]