Document that NOT NULL domain constraints are not always honored.

diff --git a/doc/src/sgml/ref/create_domain.sgml b/doc/src/sgml/ref/create_domain.sgml
index 637326b58bbbe74bb45ec110377e7a3bd20f6211..eb3bb3d98232a7e85f4a2d6eb10974fdcb2e757e 100644 (file)
--- a/doc/src/sgml/ref/create_domain.sgml
+++ b/doc/src/sgml/ref/create_domain.sgml
@@ -1,5 +1,5 @@
 
 
@@ -121,7 +121,12 @@ CREATE DOMAIN name [ AS ] 
       NOT NULL
       
        
-        Values of this domain are not allowed to be null.
+        Values of this domain are normally prevented from being null.
+        However, it is still possible for a domain with this constraint
+        to take a null value if it is assigned a matching domain type
+        that has become null, e.g. via a LEFT OUTER JOIN, or
+        INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM
+        tab WHERE false)).