Re: In-doubt window

"Jeroen T. Vermeulen" <jtv(at)xs4all(dot)nl> writes:
> Some of you may remember that we discussed libpqxx's higher-level
> support for transactionality.

> [complicated scheme]

Another way you can look to see if a transaction has completed is to see
if there is still an ExclusiveLock record for it in pg_locks. For this
you need to know the transaction's internal XID, but you can get that
from the XMAX of the log record it deleted. In other words:

insert log record;
begin;
delete log record;
<<payload work>>
commit;

On reconnect:

log record not present: it committed
else, read XMAX from log record
zero -> failed before the delete :-(
not zero -> look for match in pg_locks
found -> transaction still in progress, wait
not found -> it aborted

This is all pretty messy though. Seems like it would be a whole lot
easier if the backend exposed ways to (a) obtain your current
transaction number and (b) inquire directly about the commit status of
a transaction. However, with the present transaction infrastructure
(b) would only work for a limited time window after the original
transaction (until the corresponding clog space got recycled). So that
might or might not be good enough for your purposes.

regards, tom lane