Horton

Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an infinite loop

Tomas Vondra <tomas.vondra@2ndquadrant.com>

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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: "Todd A. Cook" <tcook@blackducksoftware.com>, Tom Lane <tgl@sss.pgh.pa.us>, Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Andres Freund <andres@anarazel.de>, PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Date: 2017-11-28T15:02:19Z
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

Commits

  1. Improve bit perturbation in TupleHashTableHash.

  2. Prevent growth of simplehash tables when they're "too empty".

  3. Add stack-overflow guards in set-operation planning.