Re: [HACKERS] bug in numeric_power() function - Mailing list pgsql-patches

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  <bruce@momjian.us>        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 ----