From: Neil Conway <neilc@samurai.com>
Date: Wed, 19 Sep 2007 22:31:48 +0000 (+0000)
Subject: Prevent corr() from returning the wrong results for negative correlation
X-Git-Tag: REL8_3_BETA1~149
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=bbf4fdc2538097bb3103806e1419ceef1f289203;p=thirdparty%2Fpostgresql.git

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 7a66bd4c56b..53bfd321d7d 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 1.150 2007/06/05 21:31:06 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/float.c,v 1.151 2007/09/19 22:31:48 neilc Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2274,8 +2274,7 @@ 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));
 }
 
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