Improve our private implementation of cbrt() to give results of the

accuracy expected by the regression tests.  Per suggestion from
Martijn van Oosterhout.

diff --git a/src/backend/utils/adt/float.c b/src/backend/utils/adt/float.c
index fb37e36624e6c5dfcddb021aefa1eb5bf7924119..d80b3d4ef49ac596a3df6aba47f045dd0286953a 100644 (file)
--- a/src/backend/utils/adt/float.c
+++ b/src/backend/utils/adt/float.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/utils/adt/float.c,v 1.115 2005/10/15 02:49:28 momjian Exp $
+ *   $PostgreSQL: pgsql/src/backend/utils/adt/float.c,v 1.115.2.1 2006/04/24 20:36:41 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2321,11 +2321,22 @@ float84ge(PG_FUNCTION_ARGS)
 /* ========== PRIVATE ROUTINES ========== */
 
 #ifndef HAVE_CBRT
+
 static double
 cbrt(double x)
 {
    int         isneg = (x < 0.0);
-   double      tmpres = pow(fabs(x), (double) 1.0 / (double) 3.0);
+   double      absx = fabs(x);
+   double      tmpres = pow(absx, (double) 1.0 / (double) 3.0);
+
+   /*
+    * The result is somewhat inaccurate --- not really pow()'s fault,
+    * as the exponent it's handed contains roundoff error.  We can improve
+    * the accuracy by doing one iteration of Newton's formula.  Beware of
+    * zero input however.
+    */
+   if (tmpres > 0.0)
+       tmpres -= (tmpres - absx/(tmpres*tmpres)) / (double) 3.0;
 
    return isneg ? -tmpres : tmpres;
 }