Clarify that cross-row constraints are unsupported

Maybe we'll implement them later, or maybe not, but let's make the statu
quo clear for now.

Author: Lætitia Avrot, Patrick Francelle
Reviewers: too many to list
Discussion: https://postgr.es/m/CAB_COdhUuzNFOJfc7SNNso5rOuVA3ui93KMVunEM8Yih+K5A6A@mail.gmail.com

doc/src/sgml/ddl.sgml

@@ -403,6 +403,33 @@ CREATE TABLE products (
     ensure that a column does not contain null values, the not-null
     constraint described in the next section can be used.
    </para>
+
+   <note>
+    <para>
+     <productname>PostgreSQL</productname> does not support
+     <literal>CHECK</literal> constraints that reference table data other than
+     the new or updated row being checked.  While a <literal>CHECK</literal>
+     constraint that violates this rule may appear to work in simple
+     tests, it cannot guarantee that the database will not reach a state
+     in which the constraint condition is false (due to subsequent changes
+     of the other row(s) involved).  This would cause a database dump and
+     reload to fail.  The reload could fail even when the complete
+     database state is consistent with the constraint, due to rows not
+     being loaded in an order that will satisfy the constraint.  If
+     possible, use <literal>UNIQUE</literal>, <literal>EXCLUDE</literal>,
+     or <literal>FOREIGN KEY</literal> constraints to express
+     cross-row and cross-table restrictions.
+    </para>
+
+    <para>
+     If what you desire is a one-time check against other rows at row
+     insertion, rather than a continuously-maintained consistency
+     guarantee, a custom <link linkend="triggers">trigger</link> can be used
+     to implement that.  (This approach avoids the dump/reload problem because
+     <application>pg_dump</application> does not reinstall triggers until after
+     reloading data, so that the check will not be enforced during a dump/reload.)
+    </para>
+   </note>
   </sect2>
 
   <sect2>

doc/src/sgml/ref/create_table.sgml

@@ -754,7 +754,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
      <para>
       Currently, <literal>CHECK</literal> expressions cannot contain
       subqueries nor refer to variables other than columns of the
-      current row.  The system column <literal>tableoid</literal>
+      current row (see <xref linkend="ddl-constraints-check-constraints"/>).
+      The system column <literal>tableoid</literal>
       may be referenced, but not any other system column.
      </para>