From: Tom Lane <tgl@sss.pgh.pa.us>
To: Michael Paquier <michael@paquier.xyz>
cc: todoubaba@gmail.com, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #16856: Crash when add "_RETURN" rule on child table
In-reply-to: <YB6QA1T0X//P+99P@paquier.xyz>
References: <16856-0363e05c6e1612fd@postgresql.org>
 <YB6QA1T0X//P+99P@paquier.xyz>
Comments: In-reply-to Michael Paquier <michael@paquier.xyz>
	message dated "Sat, 06 Feb 2021 21:50:24 +0900"
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Content-ID: <191746.1612632025.1@sss.pgh.pa.us>
Date: Sat, 06 Feb 2021 12:20:25 -0500
Message-ID: <191747.1612632025@sss.pgh.pa.us>
Precedence: bulk

Michael Paquier <michael@paquier.xyz> writes:
> On Sat, Feb 06, 2021 at 07:46:14AM +0000, PG Bug reporting form wrote:
>> create table parent(a text);
>> create table child() inherits (parent);
>> create or replace rule "_RETURN" as
>> on select
>> to child
>> do instead
>> select *
>> from (values('x')) as t(a);
>> 
>> select * from parent;

> Yes, reproduced here.  This crashes as the relcache entry of the child
> relation is not getting its rd_tableam initialized.

I don't think it's unreasonable for the code to assume that child
relations are tables and not views.  If you did the equivalent

regression=# create table parent(a text);
CREATE TABLE
regression=# create view child as select *
regression-#     from (values('x')) as t(a);
CREATE VIEW
regression=# alter table child inherit parent;
ERROR:  "child" is not a table or foreign table

or for that matter

regression=# alter table parent inherit child;
ERROR:  "child" is not a table or foreign table

So my take is that this is an oversight in the CREATE RULE logic
that allows converting a table to a view: if it has inheritance
parents or children we must disallow doing that.

I'd be inclined to back-patch further than 12, too.  Even if
there's not an obvious instant crash in those branches, it's
unlikely that their behavior is entirely sane.

			regards, tom lane