Date: Wed, 6 Dec 2017 12:46:16 -0800
From: Andres Freund <andres@anarazel.de>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: "Todd A. Cook" <tcook@blackducksoftware.com>,
	Tom Lane <tgl@sss.pgh.pa.us>,
	Thomas Munro <thomas.munro@enterprisedb.com>,
	PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Subject: Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an
 infinite loop
Message-ID: <20171206204616.wcect35fuj5gjde2@alap3.anarazel.de>
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On 2017-12-06 21:38:42 +0100, Tomas Vondra wrote:
> It's one thing when the hash table takes longer to lookup something or

longer aka "forever".


> when it consumes a bit more memory. Say, ~2x more than needed, give or
> take. I'm perfectly fine with that, particularly when it's a worst-case
> evil data set like this one.

I think the way to prevent that kind of attack is to add randomization.


> FWIW I've constructed the data sets for two reasons - to convince myself
> that my understanding of the simplehash code is correct, and to provide
> a data set triggering the other growth condition in simplehash code. My
> understanding is that if we stop growing the table after the load factor
> drops below some threshold (as TL proposed earlier in this thread), it
> should address both of these cases.

Yea, I'm not adverse to adding a few stopgaps that break in a less
annoying manner. WAll I'm saying is that I don't think we need to be
super concerned about this specific way of breaking things.

Greetings,

Andres Freund