Re: pg_dump --snapshot

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On 05/07/2013 06:37 AM, Joe Conway wrote:
> On 05/06/2013 03:00 PM, Stephen Frost wrote:
> > For example, I'm not sure that we need more information in the
> > WAL.. What we need is a way to tell VACUUM to skip over 'recently
> > modified' records and not mark them as dead until some time has
> > passed.  This is essentially what we're already doing with the
> > feedback mechanism in replication, isn't it?  Would it be possible
> > to make that a simple timer instead of requiring a replication
> > system which is feeding back that information based on queries
> > which are running on the replica?
>
> This sounds a lot like having an "idle in transaction" session hanging
> around. Could we use something akin to a prepared transaction to
> preserve the ability to use the snapshot? Maybe with an (optional)
> expiration timestamp?

I was thinking the same thing myself but assuming it was just another
one of my impractical ideas that're borne of lack of in-depth
understanding of the problem. A "lock holder" of some kind that's
independent of the session seems to be required here, either associated
directly with the snapshot or created after it and managed
independently. A prepared transaction created shortly after the commit
the snapshot refers to that holds all the required locks would seem to
serve the required purpose. A bit of a hack, but enough to make this
sane to use until/unless someone has the time/funding to do the major
rework needed to make this work "right".

- -- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services
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