Re: In PG12, query with float calculations is slower than PG11

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2020-02-13 16:25:25 +0000, Emre Hasegeli wrote:
>> And also this commit is changing the usage of unlikely() to cover
>> the whole condition. Using it only for the result is not semantically
>> correct. It is more than likely for the result to be infinite when
>> the input is, or it to be 0 when the input is.

> I'm not really convinced by this fwiw.

> Comparing

> if (unlikely(isinf(result) && !isinf(num)))
> float_overflow_error();

> with

> if (unlikely(isinf(result)) && !isinf(num))
> float_overflow_error();

> I don't think it's clear that we want the former. What we want to
> express is that it's unlikely that the result is infinite, and that the
> compiler should optimize for that. Since there's a jump involved between
> the check for isinf(result) and the one for !isinf(num), we want the
> compiler to implement this so the non-overflow path follows the first
> check, and the rest of the check is later.

Yeah, I was wondering about that. I'll change it as you suggest.

regards, tom lane