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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>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
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 09:19:23 -0800
Message-ID: <20171206171923.dg7phzfh7nb4lou3@alap3.anarazel.de> (raw)
In-Reply-To: <25629.1512580464@sss.pgh.pa.us>
References: <7a22f496-92b5-d199-984e-183965918304@blackducksoftware.com>
	<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>
	<219b37b2-d740-d04a-203a-49ae422028f7@blackducksoftware.com>
	<c15378b3-d4aa-a5c0-5538-dd52904dcbc6@2ndquadrant.com>
	<1773c302-2fb1-749f-8ccc-62b23617c5ac@blackducksoftware.com>
	<25629.1512580464@sss.pgh.pa.us>

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.c...


> 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




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