On 01/14/2014 11:22 PM, Peter Geoghegan wrote:
The problem was that
inserted-then-deleted-in-same-xact tuples (both regular and promise)
were invisible to all xacts' dirty snapshots, when they should have
only been invisible to the deleting xact's dirty snapshot.


So it isn't
obvious to me how you interlock things such that another xact doesn't
incorrectly decide that it has to wait on what is really a promise
tuple's xact for the full duration of that xact, having found no
speculative insertion token to ShareLock (which implies unprincipled
deadlocking), while simultaneously having other sessions not fail to
see as dirty-visible a same-xact-inserted-deleted non-promise tuple
(thereby ensuring those other sessions correctly conclude that it is
necessary to wait for the end of the xmin/xmax xact). If you set the
xmin to invalid too late, it doesn't help any existing waiters.

If a backend finds no speculative insertion token to ShareLock, then it really isn't a speculative insertion, and the process should sleep on the xid as usual.

Once we remove the modification in HeapTupleSatisfiesDirty() that made it return false when xmin == xmax, the problem that arises is that another backend that sees the killed tuple incorrectly determines that it has to wait for it that transaction to finish, even though it was a speculatively inserted tuple that was killed, and hence can be ignored. We can avoid that problem by setting xmin to invalid, or otherwise marking the tuple as dead.

Attached is a patch doing that, to again demonstrate what I mean. I'm not sure if setting xmin to invalid is really the best way to mark the tuple dead; I don't think a tuple's xmin can currently be InvalidTransaction under any other circumstances, so there might be some code out there that's not prepared for it. So using an infomask bit might indeed be better. Or something else entirely.

- Heikki

Attachment: speculative-insertions-revisions.2014_01_15.patch.gz
Description: GNU Zip compressed data

Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:

Reply via email to