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From: Heikki Linnakangas <hlinnakangas@vmware.com>
To: Andres Freund <andres@2ndquadrant.com>
Cc: Rajeev rastogi <rajeev.rastogi@huawei.com>
Cc: Ants Aasma <ants@cybertec.at>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Cc: Markus Wanner <markus@bluegap.ch>
Subject: Re: Proposal for CSN based snapshots
Date: Mon, 12 May 2014 19:14:55 +0300
Message-ID: <5370F37F.4060104@vmware.com> (raw)
In-Reply-To: <20140512152628.GC9619@awork2.anarazel.de>
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On 05/12/2014 06:26 PM, Andres Freund wrote:
>> >With the new "commit-in-progress" status in clog, we won't need the
>> >sub-committed clog status anymore. The "commit-in-progress" status will
>> >achieve the same thing.
> Wouldn't that cause many spurious waits? Because commit-in-progress
> needs to be waited on, but a sub-committed xact surely not?

Ah, no. Even today, a subxid isn't marked as sub-committed, until you 
commit the top-level transaction. The sub-commit state is a very 
transient state during the commit process, used to make the commit of 
the sub-transactions and the top-level transaction appear atomic. The 
commit-in-progress state would be a similarly short-lived state. You 
mark the subxids and the top xid as commit-in-progress just before the 
XLogInsert() of the commit record, and you replace them with the real 
LSNs right after XLogInsert().

- Heikki


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