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Re: Hot standby, running xacts, subtransactions

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Hot standby, running xacts, subtransactions
Date: 2009-02-25 21:05:40
Message-ID: 1235595940.16176.347.camel@ebony.2ndQuadrant (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Wed, 2009-02-25 at 22:39 +0200, Heikki Linnakangas wrote:

> When we take the snapshot of running transactions in the master, in 
> GetRunningTransactionData(), it only includes top-level xids and those 
> subxids that are in the subxid caches. Overflowed subxids are not 
> included. Isn't that a problem? When the standby initializes the 
> recovery procs using the running xacts information, pg_subtrans doesn't 
> isn't set for the overflowed xids, because that information is not 
> included in the WAL record. If you're lucky, the information is there 
> already, but we don't generally guarantee pg_subtrans to survive crash 
> or restart.

That is exactly the reason why we don't treat an overflowed snapshot as
a valid starting point.

 Simon Riggs 
 PostgreSQL Training, Services and Support

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