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Re: Disabled features on Hot Standby

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Disabled features on Hot Standby
Date: 2012-01-14 05:44:06
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Lists: pgsql-hackers
On Fri, Jan 13, 2012 at 12:18 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Fri, Jan 13, 2012 at 5:08 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> Also, what happens if an all-visible bit gets set on the standby
>> through some other mechanism - e.g. restored from an FPI or
>> XLOG_HEAP_NEWPAGE?  I'm not sure whether we ever do an FPI of the
>> visibility map page itself, but we certainly do it for the heap pages.
>>  So it might be that this infrastructure would (somewhat bizarrely)
>> trust the visibility map bits but not the PD_ALL_VISIBLE bits.  I'm
>> hoping Heikki or Tom will comment on this thread, because I think
>> there are a bunch of subtle issues here and that we could easily screw
>> it up by trying to plow through the problem too hastily.
> An FPI can't change the all visible flag. If it did, it would imply
> that we'd skipped generation or replay of an XLOG_HEAP2_VISIBLE
> record, or that we're doing crash recovery/inital startup and HS is
> not yet enabled.

Interesting.  I hadn't considered that point.

Robert Haas
The Enterprise PostgreSQL Company

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