Foreign Key issue - pg_shadow

I suppose the below reference isn't intended to be done. But, I'd
like to extend the pg_shadow table to store information about the
database users and some of their actions (a history of changes).

Is it safe to add a UNIQUE constraint to the usesysid or usename
columns?

temp=# \d pg_shadow

Table "pg_shadow"

Attribute | Type | Modifier

-------------+---------+----------

usename | name |

usesysid | integer |

usecreatedb | boolean |

usetrace | boolean |

usesuper | boolean |

usecatupd | boolean |

passwd | text |

valuntil | abstime |

temp=#

temp=# CREATE TABLE users (

temp(# pg_username name NOT NULL

temp(# REFERENCES pg_shadow(usesysid)

temp(# ON UPDATE CASCADE

temp(# ON DELETE RESTRICT

temp(# );

NOTICE: CREATE TABLE will create implicit trigger(s) for FOREIGN KEY
check(s)

ERROR: UNIQUE constraint matching given keys for referenced table
"pg_shadow" not found

temp=#

--
Rod Taylor

There are always four sides to every story: your side, their side, the
truth, and what really happened.