diff options
| author | Tom Lane <tgl@sss.pgh.pa.us> | 2006-04-24 20:36:32 +0000 |
|---|---|---|
| committer | Tom Lane <tgl@sss.pgh.pa.us> | 2006-04-24 20:36:32 +0000 |
| commit | 332ea60d2378ef8d567c93502fa7b95eccd962e5 (patch) | |
| tree | 99aecc056139e4a60200d9cc8b18bf7c7f6a963d /src/backend/utils/adt/float.c | |
| parent | 7e97b419df2001efe8cf4880ca547f814a5dd7d2 (diff) | |
| download | postgresql-332ea60d2378ef8d567c93502fa7b95eccd962e5.tar.gz postgresql-332ea60d2378ef8d567c93502fa7b95eccd962e5.zip | |
Improve our private implementation of cbrt() to give results of the
accuracy expected by the regression tests. Per suggestion from
Martijn van Oosterhout.
Diffstat (limited to 'src/backend/utils/adt/float.c')
| -rw-r--r-- | src/backend/utils/adt/float.c | 15 |
1 files changed, 13 insertions, 2 deletions
diff --git a/src/backend/utils/adt/float.c b/src/backend/utils/adt/float.c index 3382b86ba0d..b5147c9aa45 100644 --- 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 1.123 2006/03/11 01:19:22 neilc Exp $ + * $PostgreSQL: pgsql/src/backend/utils/adt/float.c,v 1.124 2006/04/24 20:36:32 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -2373,11 +2373,22 @@ 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; } |