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From: Andres Freund <andres@anarazel.de>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Todd A. Cook <tcook@blackducksoftware.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Subject: Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an infinite loop
Date: Wed, 6 Dec 2017 12:46:16 -0800
Message-ID: <20171206204616.wcect35fuj5gjde2@alap3.anarazel.de> (raw)
In-Reply-To: <06523583-b9c5-1424-ef0a-1fc8063e9ed7@2ndquadrant.com>
References: <20171127221041.mezzysc4zcfsqkyc@alap3.anarazel.de>
	<CAEepm=3SC9zYLKhvtZbsnwf9vEpi96WCGz1rdGLofuzBau=fqA@mail.gmail.com>
	<17357.1511841783@sss.pgh.pa.us>
	<9cd29b81-7ba1-91a2-f491-9014f4ac87e6@blackducksoftware.com>
	<2a76784c-06cb-46ec-35be-9895d144b839@2ndquadrant.com>
	<263b03b1-3e1c-49ca-165a-8ac6751419c4@2ndquadrant.com>
	<20171206193545.3imvbfwjgrmuho3s@alap3.anarazel.de>
	<ebe8b411-006e-7863-3c60-67ddafe1c636@2ndquadrant.com>
	<790BC80E-D8E5-499B-A1E3-8EE0752B53ED@anarazel.de>
	<06523583-b9c5-1424-ef0a-1fc8063e9ed7@2ndquadrant.com>

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




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