Date: Wed, 10 Jul 2019 23:26:09 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andreas Seltenreich <seltenreich@gmx.de>,
	pgsql-hackers@lists.postgresql.org,
	Tomas Vondra <tomas.vondra@postgresql.org>
Subject: Re: [sqlsmith] Crash in mcv_get_match_bitmap
Message-ID: <20190710212609.5mrl7hltbozoxpq7@development>
References: <8736jdhbhc.fsf@ansel.ydns.eu>
 <15097.1562792274@sss.pgh.pa.us>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Disposition: inline
In-Reply-To: <15097.1562792274@sss.pgh.pa.us>
User-Agent: NeoMutt/20180716-1444-295967-dirty
Precedence: bulk

On Wed, Jul 10, 2019 at 04:57:54PM -0400, Tom Lane wrote:
>Andreas Seltenreich <seltenreich@gmx.de> writes:
>> running sqlsmith on the regression database of REL_12_STABLE at
>> ff597b656f yielded a crash in mcv_get_match_bitmap.  I can reproduce it
>> with the following query on the regression database:
>>     select filler1 from mcv_lists where a is not null and (select 42) <= c;
>
>Seems to be the same problem I just complained of in the other
>thread: mcv_get_match_bitmap has an untenable assumption that
>"is a pseudoconstant" means "is a Const".
>

Yeah, that's a bug. Will fix (not sure how yet).

BTW which other thread? I don't see any other threads mentioning this
function.

>I notice it's also assuming that the Const must be non-null.
>It's not really easy to crash it that way, because if you just
>write "null <= c" that'll get reduced to constant-NULL earlier.
>But I bet there's a way.
>

Hmmm, I did not think of that.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services