Re: UUID data format 4x-4x-4x-4x-4x-4x-4x-4x

James Mansion wrote:
> Mark Mielke wrote:
>> I recall there being a measurable performance difference between the
>> most liberal parser, and the most optimized parser, back when I wrote
>> one for PostgreSQL. I don't know how good the one in use for
>> PostgreSQL 8.3 is. As to whether the cost is noticeable to people or
>> not - that depends on what they are doing. The problem is that a UUID
>> is pretty big, and parsing it liberally means a loop.
>>
> It just seems odd - I would have thought one would use re2c or ragel
> to generate something and the performance would essentially be O[n] on
> the input length in characters - using either a collection of allowed
> forms or an engine that normalises case and discards the '-'
> characters between any hex pairs.

Instruction level parallelism allows for multiple hex values to be
processed in parallel, whereas a loop relies on branch prediction and
speculative load and store? :-)

The liberal version is difficult to unroll. The strict version is easy
to unroll.

> So yes these would have a control loop. Is that so bad?
>
> Either way its hard to imagine how parsing a string of this length
> could create a measurable performance issue compared to what will
> happen with the value post parse.

I think so too.

Cheers,
mark

--
Mark Mielke <mark(at)mielke(dot)cc>