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Re: wCTE: why not finish sub-updates at the end, not the beginning?

From: David Fetter <david(at)fetter(dot)org>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Marko Tiikkaja <marko(dot)tiikkaja(at)cs(dot)helsinki(dot)fi>,pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: wCTE: why not finish sub-updates at the end, not the beginning?
Date: 2011-02-26 02:46:01
Message-ID: 20110226024601.GA27388@fetter.org (view raw or flat)
Thread:
Lists: pgsql-hackers
On Fri, Feb 25, 2011 at 09:58:36AM -0500, Tom Lane wrote:
> I had what seems to me a remarkably good idea, though maybe someone else
> can spot a problem with it.  Given that we've decided to run the
> modifying sub-queries all with the same command counter ID, they are
> logically executing "in parallel".  The current implementation takes no
> advantage of that fact, though: it's based around the idea of running
> the updates strictly sequentially.  I think we should change it so that
> the updates happen physically, not only logically, concurrently.
> Specifically, I'm imagining getting rid of the patch's additions to
> InitPlan and ExecutePlan that find all the modifying sub-queries and
> force them to be cycled to completion before the main plan runs.
> Just run the main plan and let it pull tuples from the CTEs as needed.
> Then, in ExecutorEnd, cycle any unfinished ModifyTable nodes to
> completion before shutting down the plan.  (In the event of an error,
> we'd never get to ExecutorEnd, but it doesn't matter since whatever
> updates we did apply are nullified anyhow.)

What's the effect, if any, on CTEs that depend on each other
explicitly?

Cheers,
David.
-- 
David Fetter <david(at)fetter(dot)org> http://fetter.org/
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Subject: Re: wCTE: about the name of the feature
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