Re: Proposal for CSN based snapshots

On 2014-05-12 19:14:55 +0300, Heikki Linnakangas wrote:
> 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().

Ah, right. Forgot that detail...

Greetings,

Andres Freund

-- Andres Freund	                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services