Add documentation clarification for IS [NOT] NULL and row-valued

expressions.

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 0811a282db2ee0a8fe00bf60d584a86a5dada0dc..097e0560ce22e997f4e9686599df739f4d2d9048 100644 (file)
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -1,4 +1,4 @@
-
+
 
  
   Functions and Operators
@@ -344,7 +344,11 @@
     IS NULL is true when the row expression itself is null
     or when all the row's fields are null, while
     IS NOT NULL is true when the row expression itself is non-null
-    and all the row's fields are non-null.
+    and all the row's fields are non-null.  Because of this behavior,
+    IS NULL and IS NOT NULL do not always return
+    inverse results for row-valued expressions, i.e. a row-valued
+    expression that contains both NULL and non-null values will return false
+    for both tests.
     This definition conforms to the SQL standard, and is a change from the
     inconsistent behavior exhibited by PostgreSQL
     versions prior to 8.2.