Re: nested xacts and phantom Xids

On Sat, Jun 26, 2004 at 07:56:09PM -0400, Tom Lane wrote:
> Do we really need SubtransCutoffXid? AFAICS the reason for having it is
> only this claim in RecordTransactionCommit:
>
> * We can't mark committed subtransactions as fully committed,
> * because concurrent transactions would see them as committed
> * and not as in-progress. Leave them as "subcommitted" until
> * the parent transaction is below OldestXmin, per VACUUM.

Right, that's the only point where it's used. I don't know clearly if
some kind of mechanism will be needed to handle SUBTRANS COMMIT states
in pg_clog that were left behind by a crashed subtransaction though.

> but I think this is dead wrong. As long as we mark the parent committed
> first, there is no race condition. tqual tests that are using snapshots
> will need to recognize that the subtransaction is a member of one of the
> snapshotted main XIDs, and those that are not will think committed is
> committed. So I want to mark subtransactions fully committed in
> RecordTransactionCommit, and lose SubtransCutoffXid. Comments?

Yes, sounds good.

> BTW, it would help to know what parts of the patch you intend to work on
> over the next couple of days. I'm reviewing and editorializing right
> now with intent to commit soon, so it would be good if we can avoid
> tromping on each others' feet.

This is really excellent news.

I'll work on adding the Xid-cache to PGPROC and using that in
TransactionIdIsInProgress and the tqual routines. If you want to work
on that let me know and I'll handle things like the password file, local
bufmgr refcount, etc.

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
FOO MANE PADME HUM