Robert Haas writes:
> On Mon, Aug 1, 2016 at 11:27 AM, Tom Lane wrote:
>> Perhaps, but this'd break existing hash indexes. That might not be
>> a fatal objection, but if we're going to put users through that
>> I'd like to think a little bigger in terms of the benefits we get.
>> I've thought fo
On Mon, Aug 1, 2016 at 7:24 AM, Dean Rasheed wrote:
> On 1 August 2016 at 08:19, Greg Stark wrote:
>> Surely combining multiple hashes is the same problem as hashing a block of
>> memory? Shouldn't we just use the same algorithm as hash_any()?
>
> Yes, I imagine that should work well. I suspect t
On Mon, Aug 1, 2016 at 11:27 AM, Tom Lane wrote:
> Dean Rasheed writes:
>> On that subject, while looking at hashfunc.c, I spotted that
>> hashint8() has a very obvious deficiency, which causes disastrous
>> performance with certain inputs:
>
> Well, if you're trying to squeeze 64 bits into a 32-
Dean Rasheed writes:
> On that subject, while looking at hashfunc.c, I spotted that
> hashint8() has a very obvious deficiency, which causes disastrous
> performance with certain inputs:
Well, if you're trying to squeeze 64 bits into a 32-bit result, there
are always going to be collisions somewh
Greg Stark writes:
> I was originally going to suggest using a crc to combine but iirc we
> changed hash_any() a while back and decided against using crc. I don't know
> if that was wise but wouldn't want to suggest relitigating that.
Nah, CRCs are designed to solve a different problem, ie detec
On 1 August 2016 at 08:19, Greg Stark wrote:
> Surely combining multiple hashes is the same problem as hashing a block of
> memory? Shouldn't we just use the same algorithm as hash_any()?
>
Yes, I imagine that should work well. I suspect that Thomas's proposal
would also probably work well, as wo
Surely combining multiple hashes is the same problem as hashing a block of
memory? Shouldn't we just use the same algorithm as hash_any()?
I was originally going to suggest using a crc to combine but iirc we
changed hash_any() a while back and decided against using crc. I don't know
if that was w
On Mon, Aug 1, 2016 at 4:34 AM, Tom Lane wrote:
> Thomas Munro writes:
>> 2. I suspect that this algorithm for combining hashes is weak, and
>> could amplify weaknesses in the hash functions feeding it.
>
> Very possibly, but ...
Concrete example: suppose a clever data type author defines a per
Thomas Munro writes:
> 2. I suspect that this algorithm for combining hashes is weak, and
> could amplify weaknesses in the hash functions feeding it.
Very possibly, but ...
> Compare
> Boost's hash_combine, which does some more work before XORing:
> seed ^= hash_value(v) + 0x9e3779b9 + (s
