Richard Wang wrote:
> I expected 0 ^ 123.3 to be 0, but it reported error as follows
>
> postgres=# select 0 ^ 123.3;
> ERROR: cannot take logarithm of zero
>
> I find that there is a bug in numeric_power() function
> the function caculates a ^ b based on the algorithm e ^ (lna * b)
> as you see, ln0 is not valid
I have developed the attached patch which fixes 0 ^ 123.3. It also
fixes the case for 0 ^ 0.0 so it returns 1 instead of an error --- see
the C comment for why one is the proper return value. float pow()
already returned one in this case:
test=> select 0 ^ 0;
?column?
----------
1
(1 row)
test=> select 0 ^ 0.0;
?column?
----------
1
(1 row)
test=> select 0 ^ 3.4;
?column?
----------
1
(1 row)
--
Bruce Momjian <[EMAIL PROTECTED]> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
Index: src/backend/utils/adt/numeric.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/utils/adt/numeric.c,v
retrieving revision 1.110
diff -c -c -r1.110 numeric.c
*** src/backend/utils/adt/numeric.c 21 Apr 2008 00:26:45 -0000 1.110
--- src/backend/utils/adt/numeric.c 7 May 2008 20:05:01 -0000
***************
*** 5170,5175 ****
--- 5170,5187 ----
int local_rscale;
double val;
+ /*
+ * This avoids log(0) for cases of 0 raised to a non-integer.
+ * We also treat 0 ^ 0 == 1 because it is the best value for discrete
+ * mathematics, and most programming languages do this.
+ * http://en.wikipedia.org/wiki/Exponentiation#Zero_to_the_zero_power
+ */
+ if (cmp_var(base, &const_zero) == 0)
+ {
+ set_var_from_var(&const_one, result);
+ return;
+ }
+
/* If exp can be represented as an integer, use power_var_int */
if (exp->ndigits == 0 || exp->ndigits <= exp->weight + 1)
{
***************
*** 5266,5280 ****
NumericVar base_prod;
int local_rscale;
- /* Detect some special cases, particularly 0^0. */
-
switch (exp)
{
case 0:
- if (base->ndigits == 0)
- ereport(ERROR,
- (errcode(ERRCODE_FLOATING_POINT_EXCEPTION),
- errmsg("zero raised to zero is undefined")));
set_var_from_var(&const_one, result);
result->dscale = rscale; /* no need to round */
return;
--- 5278,5286 ----
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