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Re: Disabled features on Hot Standby

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Disabled features on Hot Standby
Date: 2012-01-13 16:13:46
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Lists: pgsql-hackers
On Fri, Jan 13, 2012 at 3:22 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Fri, Jan 13, 2012 at 10:17 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> On Fri, Jan 13, 2012 at 2:03 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> Since the xmin horizon on the standby may precede the xmin horizon on
>>> the master
>> When hot_standby_feedback is turned on, the xmin of the standby is
>> provided to the master to ensure the situation you describe never
>> happens.
>> Surely that means the problem is solved, if we can confirm the setting
>> of the parameter. So this seems like a cleanup issues that can/should
>> be resolved.
> How do you respond to the concerns I raised about that approach in the
> last sentence of my previous email?

I think it should be you that comes up with a fix, not for me to
respond to your concerns about how hard it is. Many things that don't
fully work are rejected for that reason.

Having said that, I have input that seems to solve the problem.

Many WAL records have latestRemovedXid on them. We can use the same
idea with XLOG_HEAP2_VISIBLE records, so we add a field to send the
latest vacrelstats->latestRemovedXid. That then creates a recovery
snapshot conflict that would cancel any query that might then see a
page of the vis map that was written when the xmin was later than on
the standby. If replication disconnects briefly and a vimap bit is
updated that would cause a problem, just as the same situation would
cause a problem because of other record types.

If problem solved then we can enable IndexOnlyScans on standby.

 Simon Riggs         
 PostgreSQL Development, 24x7 Support, Training & Services

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