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Re: [HACKERS] WAL logging freezing

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
Cc: "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>, "Alvaro Herrera" <alvherre(at)commandprompt(dot)com>, pgsql-hackers(at)postgresql(dot)org, pgsql-patches(at)postgresql(dot)org
Subject: Re: [HACKERS] WAL logging freezing
Date: 2006-11-01 14:50:56
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Lists: pgsql-hackerspgsql-patches
"Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> Do we need another GUC? I thought your observation about a PITR slave
> having that set lower than its master still remains unresolved.

No, AFAICS that's not an issue in this design.  The facts-on-the-ground
are whatever is recorded in pg_class.relvacuumxid, and whether a
particular table has been vacuumed with a shorter or longer freeze
window doesn't affect correctness.  In particular, a slave with
ambitions towards having a shorter window would still be unable to
truncate its clog before having re-vacuumed everything.  

So, not only could we have a GUC variable, but it could be USERSET;
there's no breakage risk as long as we constrain the value range to
something sane.

It strikes me that VACUUM FREEZE could be replaced by
	SET vacuum_freeze_limit = 0
which would be a good thing because the FREEZE keyword has to be
partially reserved in this syntax, and that is contrary to spec.

			regards, tom lane

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