Re: BUG #16104: Invalid DSA Memory Alloc Request in Parallel Hash

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