Re: Plans for solving the VACUUM problem

> > Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > > Hey, I have an idea.  Can we do subtransactions as separate transactions
> > > (as Tom mentioned), and put the subtransaction id's in the WAL, so they
> > > an be safely committed/rolledback as a group?
> > 
> > It's not quite that easy: all the subtransactions have to commit at
> > *the same time* from the point of view of other xacts, or you have
> > consistency problems.  So there'd need to be more xact-commit mechanism
> > than there is now.  Snapshots are also interesting; we couldn't use a
> > single xact ID per backend to show the open-transaction state.
> 
> Yes, I knew that was going to come up that you have to add a lock to the
> pg_log that is only in affect when someone is commiting a transaction
> with subtransactions.  Normal transactions get read/sharedlock, while
> subtransaction needs exclusive/writelock.

I was wrong here.  Multiple backends can write to pg_log at the same
time, even subtraction ones.  It is just that no backend can read from
pg_log during a subtransaction commit.  Acctually, they can if the are
reading a transaction status that is less than the minium active
transaction id, see GetXmaxRecent().

Doesn't seem too bad.

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