doc: Clarify not-null constraints in information schema

Add a bit of clarification in various places that not-null constraints
are included under check constraints in the information schema.

diff --git a/doc/src/sgml/information_schema.sgml b/doc/src/sgml/information_schema.sgml
index d57a1323409afd3ad048409ebd77f1d8a263e0a5..9c39e9aab504b024f02b95e249bdebdfe6dcabef 100644 (file)
--- a/doc/src/sgml/information_schema.sgml
+++ b/doc/src/sgml/information_schema.sgml
@@ -917,6 +917,13 @@
    domain is the owner of the constraint.)
   
 
+  
+   The SQL standard considers not-null constraints to be check constraints
+   with a CHECK (column_name IS NOT
+   NULL) expression.  So not-null constraints are also included here
+   and don't have a separate view.
+  
+
       <structname>check_constraints</structname> Columns    @@ -2116,7 +2123,9 @@    columns in the current database that are used by some constraint.    Only those columns are shown that are contained in a table owned by    a currently enabled role.  For a check constraint, this view-   identifies the columns that are used in the check expression.  For+   identifies the columns that are used in the check expression.  For a+   not-null constraint, this view identifies the column that the constraint is+   defined on.  For    a foreign key constraint, this view identifies the columns that the    foreign key references.  For a unique or primary key constraint,    this view identifies the constrained columns.@@ -6856,7 +6865,7 @@ ORDER BY c.ordinal_position;        constraint_type character_data              -       Type of the constraint: CHECK,+       Type of the constraint: CHECK (includes not-null constraints),        FOREIGN KEY, PRIMARY KEY,        or UNIQUE       
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