Prevent corr() from returning the wrong results for negative correlation

values. The previous coding essentially assumed that x = sqrt(x*x), which
does not hold for x < 0.

Thanks to Jie Zhang at Greenplum and Gavin Sherry for reporting this
issue.

diff --git a/src/backend/utils/adt/float.c b/src/backend/utils/adt/float.c
index 161e02ba27303ddf8f52c87bbe76a650b1c01f68..81d724e499eed4b9fc0dd8a90af9513954cc8016 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 1.130 2006/10/05 01:40:45 tgl Exp $
+ *       $PostgreSQL: pgsql/src/backend/utils/adt/float.c,v 1.130.2.1 2007/09/19 22:31:51 neilc Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2469,8 +2469,7 @@ float8_corr(PG_FUNCTION_ARGS)
        if (numeratorX <= 0 || numeratorY <= 0)
                PG_RETURN_NULL();
 
-       PG_RETURN_FLOAT8(sqrt((numeratorXY * numeratorXY) /
-                                                 (numeratorX * numeratorY)));
+       PG_RETURN_FLOAT8(numeratorXY / sqrt(numeratorX * numeratorY));
 }
 
 Datum