Re: using xmin in a query?

On Thu, Jul 28, 2011 at 5:36 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Michael Nolan <htfoot(at)gmail(dot)com> writes:
> > It also appears you cannot group on a column of type xid.
>
> You can in 8.4 and up. Previous versions only know how to GROUP BY
> sortable columns, which requires a btree opclass, which xid doesn't
> have and really can't have because it doesn't have a linear ordering.
> There is a hash opclass for it, though, so in versions that know how to
> GROUP BY using hashing, it'll work.
>

:sigh: I thought I had done all the tests on my 9.0.4 testbed server too.
One of
these days I hope to get the production and development servers off 8.2.

I more or less understand why xid types don't have a linear ordering from
Robert Hass's tutorial at PGCON11.

So, a <> operator (either xid,xid or xid,integer) would need to be
implemented using the hash opclass, correct?

(I don't have a use case for it yet, though.)

>
> > Would adding a <> operator enable that?
>
> No, it's pretty irrelevant ...
>

OK, thanks for putting up with my noobie questions.
--
Mike Nolan