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Re: top-level DML under CTEs

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: Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: top-level DML under CTEs
Date: 2010-09-14 20:28:33
Message-ID: 4C8FDAF1.5010601@cs.helsinki.fi (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-rrreviewers
On 2010-09-14 10:51 PM, Tom Lane wrote:
> Hitoshi Harada<umi(dot)tanuki(at)gmail(dot)com>  writes:
>> 2010/9/15 Marko Tiikkaja<marko(dot)tiikkaja(at)cs(dot)helsinki(dot)fi>:
>>> In the email you referred to, Tom was concerned about the case where these
>>> WITH lists have different RECURSIVE declarations.  This patch makes both
>>> RECURSIVE if either of them is.  I can think of cases where that might lead
>>> to surprising behaviour, but the chances of any of those happening in real
>>> life seem pretty slim.
>
>> Does that cause surprising behavior?
>
> My recollection is that whether a CTE is marked RECURSIVE or not affects
> its scope of visibility, so that confusing the two cases can result in
> flat-out incorrect parser behavior.

The worst I can think of is:

CREATE TABLE foo(a int);

WITH t AS (SELECT * FROM foo)
INSERT INTO bar
WITH RECURSIVE foo (SELECT 1 AS a)
SELECT * FROM t;

t will actually be populated with the results of the CTE, not the table foo.

I don't think this is a huge problem in real life, but if someone thinks 
otherwise, I think we could just error out if the lists have a different 
RECURSIVE definition.

Anyone have a worse example?  Thoughts?


Regards,
Marko Tiikkaja

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