Log Message:
-----------
Have numeric 0 ^ 4.3 return 1, rather than an error, and have 0 ^ 0.0
return 1, rather than error.

This was already the float8 behavior.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
numeric.c (r1.110 -> r1.111)
(http://anoncvs.postgresql.org/cvsweb...1.110&r2=1.111)

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On Thu, 2008-05-08 at 19:25 +0000, Bruce Momjian wrote:
> Have numeric 0 ^ 4.3 return 1, rather than an error, and have 0 ^ 0.0
> return 1, rather than error.

A regression test for this behavior would be useful, I think.

-Neil



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Neil Conway wrote:
> On Thu, 2008-05-08 at 19:25 +0000, Bruce Momjian wrote:
> > Have numeric 0 ^ 4.3 return 1, rather than an error, and have 0 ^ 0.0
> > return 1, rather than error.
>
> A regression test for this behavior would be useful, I think.

Done, plus I wasn't happy with the original patch so I redid it to be
more modular, also attached.

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momjian@postgresql.org (Bruce Momjian) writes:
> Have numeric 0 ^ 4.3 return 1, rather than an error, and have 0 ^ 0.0
> return 1, rather than error.

This is wrongly described, and the implementation is still not correct
either, because it should throw an error for negative exponents.
Would you please *read* that wikipedia page you keep citing?
http://en.wikipedia.org/wiki/Exponen...Powers_of_zero

regards, tom lane

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Tom Lane wrote:
> momjian@postgresql.org (Bruce Momjian) writes:
> > Have numeric 0 ^ 4.3 return 1, rather than an error, and have 0 ^ 0.0
> > return 1, rather than error.
>
> This is wrongly described, and the implementation is still not correct
> either, because it should throw an error for negative exponents.
> Would you please *read* that wikipedia page you keep citing?
> http://en.wikipedia.org/wiki/Exponen...Powers_of_zero

I think this is fixed in the version I just committed:

test=> select 0 ^ (-1);
ERROR: invalid argument for power function
test=> select 0 ^ (-1.0);
ERROR: invalid argument for power function

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Bruce Momjian <bruce@momjian.us> http://momjian.us
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+ If your life is a hard drive, Christ can be your backup. +

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On Thu, 2008-05-08 at 18:34 -0400, Bruce Momjian wrote:
> Tom Lane wrote:
> > momjian@postgresql.org (Bruce Momjian) writes:
> > > Have numeric 0 ^ 4.3 return 1, rather than an error, and have 0 ^ 0.0
> > > return 1, rather than error.
> >
> > This is wrongly described, and the implementation is still not correct
> > either, because it should throw an error for negative exponents.
> > Would you please *read* that wikipedia page you keep citing?
> > http://en.wikipedia.org/wiki/Exponen...Powers_of_zero
>
> I think this is fixed in the version I just committed:
>
> test=> select 0 ^ (-1);
> ERROR: invalid argument for power function
> test=> select 0 ^ (-1.0);
> ERROR: invalid argument for power function

Hopefully this only occurs for 0 ^ (n)? A negative exponent isn't a
problem for y ^ x when y <> 0 and x < 0. Just checking you don't just
throw out an error for any negative exponent, which is what "invalid
argument" sounds like, to me.

Wikipedia says that exponentiation of zero to a negative power implies
division by zero, so shouldn't we throw a "division by zero" error?

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Simon Riggs
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Simon Riggs <simon@2ndquadrant.com> writes:
> Wikipedia says that exponentiation of zero to a negative power implies
> division by zero, so shouldn't we throw a "division by zero" error?

I think it should be a specific message like "zero raised to a negative
power is undefined". It's not like it's going to take us any extra code
to know that we are faced with that case.

BTW, I realized that SQL:2003 spells it all out for us in explicit
detail:

12)If <power function> is specified, then let NVEB be the <numeric value
expression base>, then let VB be the value of NVEB, let NVEE be the
<numeric value expression exponent>, and let VE be the value of NVEE.

Case:

a) If either VB or VE is the null value, then the result is the null value.

b) If VB is 0 (zero) and VE is negative, then an exception condition is
raised: data exception Ñ invalid argument for power function.

c) If VB is 0 (zero) and VE is 0 (zero), then the result is 1 (one).

d) If VB is 0 (zero) and VE is positive, then the result is 0 (zero).

e) If VB is negative and VE is not equal to an exact numeric value with
scale 0 (zero), then an exception condition is raised: data exception
Ñ invalid argument for power function.

f) If VB is negative and VE is equal to an exact numeric value with
scale 0 (zero) that is an even number, then the result is the result of
EXP(NVEE*LN(-NVEB))

g) If VB is negative and VE is equal to an exact numeric value with
scale 0 (zero) that is an odd number, then the result is the result of
-EXP(NVEE*LN(-NVEB))

h) Otherwise, the result is the result of
EXP(NVEE*LN(NVEB))


regards, tom lane

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Tom Lane wrote:
> Simon Riggs <simon@2ndquadrant.com> writes:
> > Wikipedia says that exponentiation of zero to a negative power implies
> > division by zero, so shouldn't we throw a "division by zero" error?
>
> I think it should be a specific message like "zero raised to a negative
> power is undefined". It's not like it's going to take us any extra code
> to know that we are faced with that case.
>
> BTW, I realized that SQL:2003 spells it all out for us in explicit
> detail:

....

> b) If VB is 0 (zero) and VE is negative, then an exception condition is
> raised: data exception Ñ invalid argument for power function.

Well, this indicates we shouldn't return "zero raised to a negative
power is undefined", but rather the power error we are giving now, or
are you saying we should return the "power" error code but an error
message mentioning zero?

> c) If VB is 0 (zero) and VE is 0 (zero), then the result is 1 (one).

I have updated the C comments to mention the spec also requires we
return 1 in this case.

C comment updated attached and applied.

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Bruce Momjian <bruce@momjian.us> http://momjian.us
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Bruce Momjian <bruce@momjian.us> writes:
>> b) If VB is 0 (zero) and VE is negative, then an exception condition is
>> raised: data exception Ñ invalid argument for power function.

> Well, this indicates we shouldn't return "zero raised to a negative
> power is undefined", but rather the power error we are giving now, or
> are you saying we should return the "power" error code but an error
> message mentioning zero?

The spec says what the SQLSTATE code should be. We have always felt
free to word the message text more specifically than that, though.

regards, tom lane

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Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
> >> b) If VB is 0 (zero) and VE is negative, then an exception condition is
> >> raised: data exception Ñ invalid argument for power function.
>
> > Well, this indicates we shouldn't return "zero raised to a negative
> > power is undefined", but rather the power error we are giving now, or
> > are you saying we should return the "power" error code but an error
> > message mentioning zero?
>
> The spec says what the SQLSTATE code should be. We have always felt
> free to word the message text more specifically than that, though.

OK, I will work on it then.

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Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +

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