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Re: wCTE behaviour

From: Marko Tiikkaja <marko(dot)tiikkaja(at)cs(dot)helsinki(dot)fi>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: David Fetter <david(at)fetter(dot)org>, Peter Eisentraut <peter_e(at)gmx(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Robert Haas <robertmhaas(at)gmail(dot)com>, Yeb Havinga <yebhavinga(at)gmail(dot)com>, Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>
Subject: Re: wCTE behaviour
Date: 2011-02-25 16:24:04
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On 2011-02-25 6:12 PM, Tom Lane wrote:
> Marko Tiikkaja<marko(dot)tiikkaja(at)cs(dot)helsinki(dot)fi>  writes:
>> Without hacking it broke when PQdescribePrepared was called on a
>> prepared query like:
>> SELECT 1;
>> Not sure if that's an actual problem, but it seemed like something worht
>> fixing.
> I can't replicate such a problem here --- do you have a concrete test
> case?  ISTM the issue would only have been a problem back when you
> were trying to generate multiple PlannedStmts from a query like the
> above.

I don't have one right now (I lost the one I had because of a hardware 
failure in a virtual machine), but I can write you one if you want to. 
But see below.

> The current implementation with everything in one plantree
> really ought to look just like a SELECT so far as the portal code
> is concerned.

The problem was that the old code was using PORTAL_MULTI_QUERY whenever 
a wCTE was present.  Are you saying that you are using 
PORTAL_ONE_SELECT?  Doesn't that have problems with triggers, for example?

>>> Also, why are we forbidding wCTEs in cursors?  Given the current
>>> definitions, that case seems to work fine too: the wCTEs will be
>>> executed as soon as you fetch something from the cursor.  Are you
>>> just worried about not allowing a case that might be hard to support
>>> later?
>> Honestly, I have no idea.  It might be a leftover from the previous
>> design.  If it looks like it's easy to support, then go for it.
> Right now I'm thinking that it is best to continue to forbid it.
> If we go over to the less-sequential implementation that I'm advocating
> in another thread, the timing of the updates would become a lot less
> predictable than I say above.  If we refuse it for now, we can always
> remove the restriction later, but the other way is more painful.

Fair enough.

Marko Tiikkaja

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