| From: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
|---|---|
| To: | "Todd A(dot) Cook" <tcook(at)blackducksoftware(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com> |
| Cc: | Andres Freund <andres(at)anarazel(dot)de>, PostgreSQL Bugs <pgsql-bugs(at)postgresql(dot)org> |
| Subject: | Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an infinite loop |
| Date: | 2017-11-28 15:02:19 |
| Message-ID: | c881db62-51a5-06e9-b961-80816f68c6f8@2ndquadrant.com |
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| Lists: | pgsql-bugs |
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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