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: Andres Freund <andres@anarazel.de>, Tom Lane <tgl@sss.pgh.pa.us>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>, Thomas Munro <thomas.munro@enterprisedb.com>, PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Date: 2017-12-06T19:04:56Z
Lists: pgsql-bugs 
On 12/06/17 12:19, Andres Freund wrote:
> On 2017-12-06 12:14:24 -0500, Tom Lane wrote:
>> Checking out hashint8() on random data shows no such obvious fault.
>> You've apparently got a data set that exposes a weakness in hashint8,
>> but it's not very clear what that is.
> 
> It's intentionally designed to cause problems afaict:
> 
> http://archives.postgresql.org/message-id/861b9f1f-cdc0-bc49-2595-80bc39c37dc3%40blackducksoftware.com

Definitely not intentionally.  My wording there could have been better;
perhaps "we expect to get lots of duplicates in that column and we then
need to find the unique values".  Eventually, a substantial number of
the duplicates will be deleted, but that's further along in the process.

-- 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.