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From: Andres Freund <andres@anarazel.de>
To: Todd A. Cook <tcook@blackducksoftware.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
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: Tue, 30 Jan 2018 11:34:53 -0800
Message-ID: <20180130193453.gprzxco2almo5lpk@alap3.anarazel.de> (raw)
In-Reply-To: <039b8fa5-8272-8b14-3710-926de5844677@blackducksoftware.com>
References: <20171206171923.dg7phzfh7nb4lou3@alap3.anarazel.de>
	<7de4b2d6-b119-7d15-04d8-4c0fcb29cbc3@2ndquadrant.com>
	<20180126234537.dyu6jwnqz7snpq7n@alap3.anarazel.de>
	<14184.1517010515@sss.pgh.pa.us>
	<20180129010213.rt3nu73ilff756f6@alap3.anarazel.de>
	<15ae5ae2-bc74-ebef-f9d6-34b16423cc04@blackducksoftware.com>
	<20180129183118.rdtm34fshwxlajzm@alap3.anarazel.de>
	<633f3730-0cba-9aed-2724-3ae4295bb0b2@blackducksoftware.com>
	<20180129192647.a6ni3jslsq5uzjkz@alap3.anarazel.de>
	<039b8fa5-8272-8b14-3710-926de5844677@blackducksoftware.com>

On 2018-01-30 13:57:44 -0500, Todd A. Cook wrote:
> Out of curiosity, I then modified hashint8() as previously described.
> With that change, run time dropped to 11 minutes (also average of 3
> runs).
> 
> FWIW, the data values in these sets are sort-of random (where I can't
> explain the "sort-of" in a public forum), but strongly biased towards
> negative infinity.  Starting again from scratch, we could probably remove
> the bias, but we have 28-30 billion of these things collected over the
> last 14 years (starting in PostgreSQL 7.4) and it's kinda tough to change
> directions at this point...

FWIW, you could just create a different hash opclass and use it for
those queries...

Greetings,

Andres Freund




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