-  <para>
-   <command>INSERT</command> with an <literal>ON CONFLICT DO UPDATE</>
-   clause is a <quote>deterministic</quote> statement.  This means
-   that the command will not be allowed to affect any single existing
-   row more than once; a cardinality violation error will be raised
-   when this situation arises.  Rows proposed for insertion should not
-   duplicate each other in terms of attributes constrained by the
-   conflict-arbitrating unique index.  Note that the ordinary rules
-   for unique indexes (and exclusion constraints) with regard to null
-   apply analogously to whether or not an arbitrating index indicates
-   if the alternative path should be taken.  This means that when a
-   null value appears in any constrained tuple's column in an
-   <command>INSERT</command> statement with <literal>ON
-   CONFLICT</literal>, rows proposed for insertion will never take the
-   alternative path (provided that the column appears in the
-   arbitrating index's definition, and provided that a <literal>BEFORE
-   ROW INSERT</literal> trigger does not make null values non-null
-   before insertion);  the statement will always insert, assuming
-   there is no unrelated error.  Note that merely locking a row with
-   <literal>ON CONFLICT DO UPDATE</> (and not updating due to the row
-   not satisfying the <literal>WHERE</> clause
-   <replaceable>condition</>) does not count towards whether or not
-   the row has been affected multiple times (and whether or not a
-   cardinality violation error is raised).  However, the
-   implementation checks for cardinality violations after locking the
-   row, and before <emphasis>considering</emphasis> updating, so a
-   cardinality violation may be raised despite the fact that the row
-   would not otherwise have gone on to be updated.