Minimum permissions required for pg_dump as a backup user

Hi All,

Does anyone know what the minimum permission is required to have
database backups run as a less privileged user?

We have tried granting 'SELECT' to our backup user, but it appears that
this is insufficient as pg_dump wants to lock things while it runs.

Is there some privilege less than 'UPDATE' that would allow the backups
to operate, but otherwise allow the backup user read-only access to the
database?

Thanks,
                    Andrew.
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