traints are ignored by the planner when considering
        <TT
CLASS="VARNAME"
>constraint_exclusion</TT
>, since it is not certain that all
        rows meet the constraint.
       </P
><P
>        The new <TT
CLASS="COMMAND"
>ALTER TABLE VALIDATE</TT
> command allows <TT
CLASS="LITERAL"
>NOT
        VALID</TT
> constraints to be checked for existing rows, after which
        they are converted into ordinary constraints.
       </P
></LI
><LI
><P
>        Allow <TT
CLASS="LITERAL"
>CHECK</TT
> constraints