Re: Numeric x^y for negative x

From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
To: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Cc: Jaime Casanova <jcasanov(at)systemguards(dot)com(dot)ec>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Dave Page <dpage(at)pgadmin(dot)org>
Subject: Re: Numeric x^y for negative x
Date: 2021-09-13 16:51:17
Message-ID: 202109131651.raa5zr4znnfk@alvherre.pgsql
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On 2021-Sep-12, Dean Rasheed wrote:

> So the fix is just to remove the upper bound on this local_rscale, as
> we do for the full-precision calculation. This doesn't impact
> performance, because it's only computing the logarithm to 8
> significant digits at this stage, and when x is very close to 1 like
> this, ln_var() has very little work to do -- there is no argument
> reduction to do, and the Taylor series terminates on the second term,
> since 1-x is so small.

I came here just to opine that there should be a comment about there not
being a clamp to the maximum scale. For example, log_var says "Set the
scales .. so that they each have more digits ..." which seems clear
enough; I think the new comment is a bit on the short side.

> Coming up with a test case that doesn't have thousands of digits is a
> bit fiddly, so I chose one where most of the significant digits of the
> result are a long way after the decimal point and shifted them up,
> which makes the loss of precision in HEAD more obvious. The expected
> result can be verified using bc with a scale of 2000.

I couldn't get bc (version 1.07.1) to output the result; it says

Runtime warning (func=(main), adr=47): non-zero scale in exponent
Runtime error (func=(main), adr=47): exponent too large in raise

Álvaro Herrera PostgreSQL Developer —

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