Re: General purpose hashing func in pgbench

From: Ildar Musin <i(dot)musin(at)postgrespro(dot)ru>
To: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: General purpose hashing func in pgbench
Date: 2018-01-10 13:35:34
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09/01/2018 23:11, Fabien COELHO пишет:
> Hello Ildar,
>> Sorry for a long delay. I've added hash() function which is just an
>> alias for murmur2. I've also utilized variable arguments feature from
>> least()/greates() functions to make optional seed parameter, but I
>> consider this as a hack.
> Patch needs a rebase after Teodor push for a set of pgbench functions.
Done. Congratulations on your patch finally being committed : )
>> Should we probably add some infrastructure for optional arguments?
> You can look at the handling of "CASE" which may or may not have an
> "ELSE" clause.
> I'd suggest you use a new negative argument with the special meaning
> for hash, and create the seed value when missing when building the
> function, so as to simplify the executor code.
Added a new nargs option -3 for hash functions and moved arguments check
to parser. It's starting to look a bit odd and I'm thinking about
replacing bare numbers (-1, -2, -3) with #defined macros. E.g.:

> Instead of 0, I'd consider providing a random default so that the
> hashing behavior is not the same from one run to the next. What do you
> think?
Makes sence since each thread is also initializes its random_state with
random numbers before start. So I added global variable 'hash_seed' and
initialize it with random() before threads spawned.
> Like the previous version, murmur2 with seed looks much more random
> than fnv with seed...

Ildar Musin
Postgres Professional:
Russian Postgres Company

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