From: Todd A. Cook <tcook@blackducksoftware.com>
To: Andres Freund <andres@anarazel.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Subject: Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an infinite loop
Date: Tue, 28 Nov 2017 16:05:26 -0500
Message-ID: <861b9f1f-cdc0-bc49-2595-80bc39c37dc3@blackducksoftware.com> (raw)
In-Reply-To: <20171128181459.d4pqmum2tjtrdwdd@alap3.anarazel.de>
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>
<8ee9458d-1556-1cf4-9cea-ff926a9a99d5@blackducksoftware.com>
<20171128181459.d4pqmum2tjtrdwdd@alap3.anarazel.de>
On 11/28/17 13:14, Andres Freund wrote:
> On 2017-11-28 09:55:13 -0500, Todd A. Cook wrote:
>> On 11/27/17 23:03, Tom Lane wrote:
>>>
>>> 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 ...
>>
>> In case it matters, the complete data set will have some outlier values with 10k to 100k
>> collisions in this column.
>
> To make sure we're on the same page, this is data intentionally created
> to have a lot of hash collisions, is that right?
More or less. It might be more accurate to say that it's created in such a way that
we expect to get lots of collisions.
-- todd
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To: pgsql-bugs@postgresql.org
Cc: tcook@blackducksoftware.com, andres@anarazel.de, tgl@sss.pgh.pa.us, thomas.munro@enterprisedb.com, tomas.vondra@2ndquadrant.com
Subject: Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an infinite loop
In-Reply-To: <861b9f1f-cdc0-bc49-2595-80bc39c37dc3@blackducksoftware.com>
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