Horton

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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: "Todd A. Cook" <tcook@blackducksoftware.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Thomas Munro <thomas.munro@enterprisedb.com>, PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Date: 2017-12-06T17:19:23Z
Lists: pgsql-bugs 
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


> In any case, the hashtable code needs to not fall over in the
> presence of a lot of collisions, regardless of the exact reason
> for there being a lot.

Yes, we need to be more resilient about it. Working on a patch.

Greetings,

Andres Freund

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.