From: Todd A. Cook <tcook@blackducksoftware.com>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Subject: Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an infinite loop
Date: Tue, 5 Dec 2017 17:33:25 -0500
Message-ID: <219b37b2-d740-d04a-203a-49ae422028f7@blackducksoftware.com> (raw)
In-Reply-To: <2a76784c-06cb-46ec-35be-9895d144b839@2ndquadrant.com>
References: <20171127185700.1470.20362@wrigleys.postgresql.org>
<fd924fd9-8f27-d945-04a9-57b503896107@2ndquadrant.com>
<7a22f496-92b5-d199-984e-183965918304@blackducksoftware.com>
<20171127221041.mezzysc4zcfsqkyc@alap3.anarazel.de>
<CAEepm=3SC9zYLKhvtZbsnwf9vEpi96WCGz1rdGLofuzBau=fqA@mail.gmail.com>
<17357.1511841783@sss.pgh.pa.us>
<9cd29b81-7ba1-91a2-f491-9014f4ac87e6@blackducksoftware.com>
<2a76784c-06cb-46ec-35be-9895d144b839@2ndquadrant.com>
On 12/05/17 17:01, Tomas Vondra wrote:
>
>
> On 12/05/2017 10:23 PM, Todd A. Cook wrote:
>> On 11/27/17 23:03, Tom Lane wrote:
>>> Thomas Munro <thomas.munro@enterprisedb.com> writes:
>>>> When SH_INSERT tries to insert that final extra value, insertdist
>>>> keeps exceeding SH_GROW_MAX_DIB (25) no matter how many times we
>>>> double the size (at least until my computer gives up, somewhere around
>>>> 11 doublings and 75GB of virtual memory). If you set SH_GROW_MAX_DIB
>>>> to 26 then it succeeds, but I guess some other attack could be crafted
>>>> for that. What is the theory behind this parameter?
>>>
>>> You beat me to it --- after looking at simplehash.h I'd guessed that
>>> either the SH_GROW_MAX_DIB or SH_GROW_MAX_MOVE code path was causing
>>> an infinite loop, but I'd not gotten to determining which one yet.
>>>
>>> I'd ask what's the theory behind SH_GROW_MAX_MOVE, as well. Neither
>>> of them are obviously loop-proof.
>>>
>>> Note that the sample data has a lot of collisions:
>>>
>>> regression=# select hashint8(val), count(*) from reproducer group by 1
>>> order by 2 desc;
>>> hashint8 | count
>>> -------------+-------
>>> 441526644 | 2337
>>> -1117776826 | 1221
>>> -1202007016 | 935
>>> -2068831050 | 620
>>> 1156644653 | 538
>>> 553783815 | 510
>>> 259780770 | 444
>>> 371047036 | 394
>>> 915722575 | 359
>>> ... etc etc ...
>>>
>>> It's evidently more complicated than just that the code fails with
>>> more than SH_GROW_MAX_DIB duplicate hashcodes, but I suspect not
>>> by a lot. There needs to be a safety valve that prevents letting
>>> the hash fill factor approach zero, which is what's happening in
>>> this test case.
>>
>> FWIW, I can also reproduce the infinite loop with 167834 unique values.
>>
>
> Unique values or unique *hash* values?
Unique values.
> Can you share the data, so that whoever fixes the bug can verify it also
> fixes your example?
Sure. It's attached.
-- todd
Attachments:
reproducer_2.pgsql (text/plain, 3.3M) download
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To: pgsql-bugs@postgresql.org
Cc: tcook@blackducksoftware.com, tomas.vondra@2ndquadrant.com, tgl@sss.pgh.pa.us, thomas.munro@enterprisedb.com, andres@anarazel.de
Subject: Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an infinite loop
In-Reply-To: <219b37b2-d740-d04a-203a-49ae422028f7@blackducksoftware.com>
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