Re: General purpose hashing func in pgbench

Hello Ildar,

> Actually the "bad" one appears in YCSB.

Fine. Then it must be kept, whatever its quality.

> But if we should choose the only one I would stick to murmur too given 
> it provides better results while having similar computational 
> complexity.

No. Keep both as there is a justification for the bad one. Just make 
"hash()" default to a good one.

>> One implementation put constants in defines, the other one uses "const
>> int". [...]
> [...] it looked ugly and hard to read (IMHO), like:
>
>     k *= MURMUR2_M;
>     k ^= k >> MURMUR2_R;
>     k *= MURMUR2_M;
>     result ^= k;
>     result *= MURMUR2_M;

Yep. The ugliness is significantly linked to the choice of name. With 
MM2_MUL and MM2_ROT ISTM that it is more readable:

>     k *= MM2_MUL;
>     k ^= k >> MM2_ROT;
>     k *= MM2_MUL;
>     result ^= k;
>     result *= MM2_MUL;

> [...] So I'd better leave it the way it is. Actually I was thinking to 
> do the same to fnv1a too : )

I think that the implementation style should be homogeneous, so I'd 
suggest at least to stick to one style.

I noticed from the source of all human knowledege (aka Wikipedia:-) that 
there seems to be a murmur3 successor. Have you considered it? One good 
reason to skip it would be that the implementation is long and complex. 
I'm not sure about a 8-byte input simplified version.

Just a question: Have you looked at SipHash24?

     https://en.wikipedia.org/wiki/SipHash

The interesting point is that it can use a key and seems somehow 
cryptographically secure, for a similar cost. However the how to decide 
for/control the key is unclear.

-- 
Fabien.