1 files changed, 17 insertions, 4 deletions
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 8504555bac..d6c28c8de3 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -1820,11 +1820,13 @@
 
   <para>
    The catalog <structname>pg_constraint</structname> stores check, primary
-   key, unique, foreign key, and exclusion constraints on tables.
+   key, unique, foreign key, exclusion constraints and additional information
+   of not-null constraints on tables.
    (Column constraints are not treated specially.  Every column constraint is
    equivalent to some table constraint.)
-   Not-null constraints are represented in the <structname>pg_attribute</>
-   catalog, not here.
+   Not-null constraints are primarily represented in the <structname>pg_attribute</>
+   catalog, but each attribute which has a not-null constraint in <structname>pg_attribute</>
+   has additional inheritance information stored in <structname>pg_constraint</>.
   </para>
 
   <para>
@@ -1873,6 +1875,7 @@
       <entry>
         <literal>c</> = check constraint,
         <literal>f</> = foreign key constraint,
+        <literal>n</> = not-null constraint
         <literal>p</> = primary key constraint,
         <literal>u</> = unique constraint,
         <literal>t</> = constraint trigger,
@@ -1993,7 +1996,8 @@
       <entry><type>int2[]</type></entry>
       <entry><literal><link linkend="catalog-pg-attribute"><structname>pg_attribute</structname></link>.attnum</></entry>
       <entry>If a table constraint (including foreign keys, but not constraint
-       triggers), list of the constrained columns</entry>
+       triggers), list of the constrained columns. This column is used to store
+       the referenced attribute number of a not-null constraint, too</entry>
      </row>
 
      <row>
@@ -2058,6 +2062,15 @@
    index.)
   </para>
 
+  <para>
+   Not-null constraints are used to store inheritance information for each not-null
+   constraint attached to a column. They shouldn't be confused with check
+   constraints, since they are handled differently. If a column in <structname>pg_attribute</>
+   has its <structfield>attnotnull</> set, there must be a matching not-null constraint
+   in <structname>pg_constraint</> with the attribute number of the column stored in
+   <structfield>conkey</>. There is always only one attribute number stored at the same time.
+  </para>
+
   <note>
    <para>
     <structfield>consrc</structfield> is not updated when referenced objects