System Tables ER / ALTER COLUMN

I was wondering if there is a ER diagram of the system tables..
As pgsql doesn't have a command to change the type of a field, I thought
to make a procedure that do that, changing the system tables.

Tks

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Nelio Alves Pereira Filho
IFX Networks - www.ifx.com.br
+55 11 3365-5863
nelio@ifx.com.br