Backup single database roles and ACLs

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Hello,

How do I backup all the roles and ACLs that have permissions to a single
database?

pg_dumpall -g does not do the trick because it dumps all the roles from
all the databases.

I've read the system catalogs documentation [1], but I didn't figure how
to relate the database name with the table names...

So far I can get all the database names, owners oid and ACLs using:

select oid,datname,datdba,datacl from pg_database;

But how do I relate pg_database.oid with pg_class (to get table ACLs)?
or maybe its related with other table that I'm missing?

TIA!

Best regards,
Rui Lopes

[1] http://www.postgresql.org/docs/8.2/static/catalogs.html
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