From: Michael Paquier <michael@paquier.xyz>
To: todoubaba@gmail.com
To: pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #16856: Crash when add "_RETURN" rule on child table
Date: Sat, 6 Feb 2021 21:50:24 +0900
Message-ID: <YB6QA1T0X//P+99P@paquier.xyz> (raw)
In-Reply-To: <16856-0363e05c6e1612fd@postgresql.org>
References: <16856-0363e05c6e1612fd@postgresql.org>
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. In 11 and older
versions we would just have assumed in this case to use heap. Here is
the top of the backtrace:
#0 table_beginscan (rel=0x7f714f574c98, snapshot=0x55e4a884b198,
nkeys=0, key=0x0) at ../../../src/include/access/tableam.h:860
860 return rel->rd_tableam->scan_begin(rel, snapshot,
nkeys, key, NULL, flags);
(gdb) p rel->rd_tableam
$1 = (const struct TableAmRoutine *) 0x0
There are two things I can see here:
1) RelationInitTableAccessMethod() does nothing for a view in 12~.
Obviously, because it has no physical storage.
2) RelationGetNumberOfBlocksInFork() now asserts when attempted on a
view.
And ~11 actually went through those code paths but silently returned
0.
Hmm. Maybe something needs to happen in the rewriter? Tweaking
directly SeqNext() would be grotty if we'd add a check for an empty
rd_tableam.
Also, the same type of issue has existed with currtid(), that got
fixed by e786be5. Tom has mentioned not long ago that we should have
a set of APIs that complain when invoked on relations without storage:
https://www.postgresql.org/message-id/14846.1586105516@sss.pgh.pa.us
While this would not fix this problem, I think that we should really
tackle that so as we can prevent crashes if a code path is missed. I
can see what needs to be done for that.
--
Michael
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To: pgsql-bugs@postgresql.org
Cc: michael@paquier.xyz, 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>
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