| From: | "Todd A(dot) Cook" <tcook(at)blackducksoftware(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, 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 21:05:26 |
| Message-ID: | 861b9f1f-cdc0-bc49-2595-80bc39c37dc3@blackducksoftware.com |
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| Lists: | pgsql-bugs |
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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