From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: "David G. Johnston" <david.g.johnston@gmail.com>,
        Vik Fearing <vik@2ndquadrant.fr>,
        Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
        denisa.cirstescu@asentinel.com,
        "pgsql-docs@postgresql.org" <pgsql-docs@postgresql.org>
Subject: Re: Undocumented behavior od DROP SCHEMA ... CASCADE
In-reply-to: <20160812222611.GA721702@alvherre.pgsql>
References: <20160812222611.GA721702@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
	message dated "Fri, 12 Aug 2016 18:26:11 -0400"
Date: Fri, 12 Aug 2016 18:58:32 -0400
Message-ID: <29747.1471042712@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-docs-owner@postgresql.org

Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> Tom Lane wrote:
>> You'd need the object locks in any case, to be sure things hold still long
>> enough for their dependencies to be examined.  It's possible a weaker lock
>> type would suffice, but I'm not sure; we generally don't require exclusive
>> lock on an object to add or remove dependencies on it.

> Of course, using a weak lock could run afoul of somebody changing the
> dependencies underneath.  But even using a stronger lock is unlikely to
> give any actual protection: in UI programs (be it GUI admin programs or
> psql), more likely than not many users are going to run a check in one
> transaction, then run the actual drop in a different transaction.

Well, obviously the output would not be authoritative about what might get
dropped in a later drop attempt.  But I'm thinking just about not getting
"cache lookup failed" or similar failures in the function itself.

Also, assuming that we did acquire full-strength locks, that would mean
that if you do
	begin;
	select pg_drop_cascades_to(foo);
	drop foo cascade;
	commit;
then in fact the function output WOULD be authoritative about what would
get dropped in the second step.  That seems like a useful property to
have, even if there are lots of production scenarios where you'd not want
to hold the locks long enough for a human to eyeball the list.  You
could imagine for instance an application quickly verifying that the
SELECT result matches a previously vetted list before barging ahead
with the DROP.  (Right now, you can approximate that sort of "safe
drop" by looking at the NOTICE output before committing --- but as you
noted, that only works up to 100 dependencies.)

			regards, tom lane


-- 
Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-docs