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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Andres Freund <andres@anarazel.de>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: James Coleman <jtc331@gmail.com>
Cc: PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Subject: Re: BUG #16104: Invalid DSA Memory Alloc Request in Parallel Hash
Date: Mon, 11 Nov 2019 00:44:38 +0100
Message-ID: <20191110234438.qwdcur3aodd7yj6d@development> (raw)
In-Reply-To: <20191110224631.uxpfccnql6pxllv2@alap3.anarazel.de>
References: <CA+hUKGJFNg0XqXiBNzOeTPsCa4tb-a00Sv5EA7jD+w26ATyUyA@mail.gmail.com>
	<CAAaqYe-VHx2O1k1x-wML3_1GHCPajugvMify=a_4cq-aHb=ZVg@mail.gmail.com>
	<CA+hUKGL0T7-Lsbp-dda5JWubAXEHMkL7j94-XW9ZhXDb4sn2+A@mail.gmail.com>
	<CAAaqYe9sMXm8e2+9n6KK2OTMdH3wBku06cpTcchu7k2wX2dAqA@mail.gmail.com>
	<20191110172925.mlkglpwzwego3wtp@development>
	<CAAaqYe_w=Q2F9tM0EyUvLn=CrwM99S3w9LE=MgghxKa6SRwdJw@mail.gmail.com>
	<CA+hUKGLyafKXBMFqZCSeYikPbdYURbwr+jP6TAy8sY-8LO0V+Q@mail.gmail.com>
	<20191110212352.5xpzddwjs6fmuv4u@development>
	<20191110215017.prjisnntwhhppnru@development>
	<20191110224631.uxpfccnql6pxllv2@alap3.anarazel.de>

On Sun, Nov 10, 2019 at 02:46:31PM -0800, Andres Freund wrote:
>Hi,
>
>On 2019-11-10 22:50:17 +0100, Tomas Vondra wrote:
>> On Sun, Nov 10, 2019 at 10:23:52PM +0100, Tomas Vondra wrote:
>> > On Mon, Nov 11, 2019 at 10:08:58AM +1300, Thomas Munro wrote:
>> > Can't we simply compute two hash values, using different seeds - one for
>> > bucket and the other for batch? Of course, that'll be more expensive.
>>
>> Meh, I realized that's pretty much just a different way to get 64-bit
>> hashes (which is what you mentioned).
>
>I'm not sure it's really the same, given practical realities in
>postgres. Right now the "extended" hash function supporting 64 bit hash
>functions is optional. So we couldn't unconditionally rely on it being
>present, even in master, unless we're prepared to declare it as
>required from now on.
>
>So computing two different hash values at the same time, by using a
>different IV and a different combine function, doesn't seem like an
>unreasonable approach.
>

True. I was commenting on the theoretical fact that computing two 32-bit
hashes is close to computing a 64-bit hash, but you're right there are
implementation details that may make it more usable in our case.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services





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