Horton

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

Todd A. Cook <tcook@blackducksoftware.com>

View thread View on postgr.es
From: "Todd A. Cook" <tcook@blackducksoftware.com>
To: Tom Lane <tgl@sss.pgh.pa.us>, Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Andres Freund <andres@anarazel.de>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Date: 2017-11-28T14:55:13Z
Lists: pgsql-bugs 
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.

-- todd

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.