On Thu, Apr 25, 2013 at 12:17 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> The cause is that each one will wait for all older snapshots to be
> gone --- and it does that before dropping its own snapshot, so that the
> other ones will see it as something to be waited out too.
This makes sense. Thank you for explaining.
> Since we know that C.I.C. executes in its own transaction, and there
> can't be more than one on the same table due to locking, it seems to me
> that it'd be safe to drop our own snapshot before waiting for other
> xacts to end. That is, we could just rearrange the last few steps in
> DefineIndex(), taking care to save snapshot->xmin before we destroy the
> snapshot so that we still have that value to pass to
> GetCurrentVirtualXIDs().
Seems reasonable to me. Looks like a fix landed in master yesterday:
http://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=c3d09b3bd23f5f65b5eb8124a3c7592dad85a50c
Many thanks to Tom and all the pgsql-hackers for all the work you do!
Paul