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Re: Writeable CTEs and empty relations

From: Marko Tiikkaja <marko(dot)tiikkaja(at)cs(dot)helsinki(dot)fi>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Writeable CTEs and empty relations
Date: 2010-02-10 20:24:01
Message-ID: 4B7315E1.3040902@cs.helsinki.fi (view raw or flat)
Thread:
Lists: pgsql-hackers
On 2010-02-10 21:59 +0200, Robert Haas wrote:
> On Wed, Feb 10, 2010 at 5:05 AM, Marko Tiikkaja
> <marko(dot)tiikkaja(at)cs(dot)helsinki(dot)fi> wrote:
>> On 2010-02-10 03:20 +0200, Tom Lane wrote:
>>> Marko Tiikkaja <marko(dot)tiikkaja(at)cs(dot)helsinki(dot)fi> writes:
>>>> On 2010-02-10 02:19 +0200, Tom Lane wrote:
>>>>> You still haven't explained why it's a good idea to change the snapshot
>>>>> after the executor has started.  Right at the moment I'm prepared to
>>>>> reject the patch on that ground alone.
>>>
>>>> The patch only touches the snapshot's curcid.  That's needed to allow
>>>> the queries see the tuples of the DML WITHs ran before that particular
>>>> query.
>>>
>>> ... and they will also see, for example, tuples inserted by triggers
>>> fired by those queries.  I thought the plan for this feature was to
>>> store and re-read the RETURNING data, not to go back and re-read the
>>> underlying tables.
>>
>> I originally suggested this approach about four months ago.  During this
>> whole time you haven't expressed any concerns about this, but suddenly
>> it's a reason to reject the patch?
>>
>> Anyway, if we want to avoid modifying the snapshot, we can't bump the
>> CID between queries.  I haven't thought this through, but it seems like
>> this could work.  However, none of the WITH queries can see the previous
>> statement's tuples, which means that someone may be suprised when they
>> try to modify the same tuples twice just to discover that only the first
>> modification took place.  I don't see this as a huge problem though.
> 
> I think that we agreed previously that running all the DML queries to
> completion before the main query, bumping the CID after each one, and
> saving the outputs, was the only workable approach.
> 
> http://archives.postgresql.org/pgsql-hackers/2009-10/msg00566.php

Hmm.  Yeah.  Didn't think about that :-(

> If the executor has buried in it the assumption that the snapshot
> can't change after startup, then does that mean that we need to start
> up and shut down the executor for each subquery?

Then I guess that's pretty much the only option.


Regards,
Marko Tiikkaja

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