Re: Question about SSI, subxacts, and aborted read-only xacts

On Mon, Sep 10, 2012 at 10:44:57PM -0700, Jeff Davis wrote:
> For the archives, and for those not following the paper in detail, there
> is one situation in which SSI will abort a read-only transaction.
>
> When there are three transactions forming a dangerous pattern where T1
> (read-only) has a conflict out to T2, and T2 has a conflict out to T3;
> and T3 is committed and T2 is prepared (for two-phase commit). In that
> situation, SSI can't roll back the committed or prepared transactions,
> so it must roll back the read-only transaction (T1).

This is true, but it isn't the only situation where a read-only
transaction can be rolled back -- this can happen even without
two-phase commit involved.

You can have a situation where two read/write transactions T2 and T3
conflict such that T2 appears to have executed first in the serial
order, but T3 commits before T2. If there's a read-only transaction T1
that takes its snapshot between when T3 and T2 commit, it can't be
allowed to read the data that the other two transactions modified: it'd
see the changes made by T3 but not T2, violating the serial order.

Given a choice, we'd prevent this by aborting one of the read/write
transactions. But if they've both already committed by the time the
read-only transaction T1 does its reads, we'd have to abort it instead.

(Note that this is still an improvement over two-phase locking, which
wouldn't allow any of the transactions to execute concurrently!)

What I was getting at in my previous mail was that there aren't any
situations where COMMIT will return a serialization failure for
a read-only transaction.

Dan

--
Dan R. K. Ports UW CSE http://drkp.net/