From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com> |
Cc: | "Florian G(dot) Pflug" <fgp(at)phlo(dot)org>, "PostgreSQL-patches" <pgsql-patches(at)postgresql(dot)org> |
Subject: | Re: Lazy xid assignment V3 |
Date: | 2007-09-03 13:56:50 |
Message-ID: | 8057.1188827810@sss.pgh.pa.us |
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Lists: | pgsql-patches |
"Heikki Linnakangas" <heikki(at)enterprisedb(dot)com> writes:
> Should there be new a log_line_prefix percent code for virtual
> transaction ids? Or should we change the meaning of %x to be virtual
> transaction id instead of the real one.
I think the latter should be sufficient, especially if we also are showing
vxid in pg_locks and pg_stat_activity.
> What the worst thing that happens if two sessions are assigned the same
> session id? Should be infrequent, but not impossible if you have some
> very long-lived connections and other clients connecting/disconnecting
> all the time.
I think this'll be all right. The only operations we actually do on
vxids is equality comparison. Even if you had a session that was 4
billion new-connections old, it should also have a pretty darn high
localvxid counter, and so a new session coming in and starting its
counter at 0 is highly unlikely to conflict. It's not impossible,
I guess, but the odds against seem so high that it's not worth adding
code to prevent it.
>> Since we didn't really reach an agreement on how xid_age should behave,
>> I've reverted it back to the original version. So with this patch,
>> xid_age will just force assignment of a xid.
> Sounds OK to me. It's not going to consume any more xids than it does
> now, and I don't think xid_age is called very often. What's it for
> anyway? To write scripts to run VACUUM before xid wrap-around?
Yeah. I think xid_age is really just a historical curiosity anyway as
of 8.2 --- there's hardly any real use-case for it.
regards, tom lane
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