From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Todd A. Cook <tcook@blackducksoftware.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, 28 Nov 2017 16:02:19 +0100
Message-ID: <c881db62-51a5-06e9-b961-80816f68c6f8@2ndquadrant.com> (raw)
In-Reply-To: <8ee9458d-1556-1cf4-9cea-ff926a9a99d5@blackducksoftware.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>
<8ee9458d-1556-1cf4-9cea-ff926a9a99d5@blackducksoftware.com>
On 11/28/2017 03:55 PM, 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.
>
In the original values? Not a big deal, I guess. It's the hashint8
collisions that's causing the infinite loop, i.e. different values with
the same hashint8 result.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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To: pgsql-bugs@postgresql.org
Cc: tomas.vondra@2ndquadrant.com, tcook@blackducksoftware.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: <c881db62-51a5-06e9-b961-80816f68c6f8@2ndquadrant.com>
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