PGDATA, PGDATA2, pg_database

Welcome
I need some information about PGDATA and PGDATA2 variable and system table
pg_database.

I'm read doc and have a question.
for example
PGDATA =/usr/data this is clouster_1
PGDATA2 =/usr2/data this is clouster_2

First question
Is pg_database i one for two clouster or one in clouster_1 and
second copy in clouster_2 with other database?

Second question.
I can run postmaster with param -D and path to databases location for example
clouster_1.
In this moment when I run select comand on pg_database I will see only
database from clouster one or database form clouster_1 and clouster_2 and I
will connect only to database from clouster_1???

Third question
What will hapend where I set variable PGDATA and PGDATA2 in my environment
and run postmaster -i -o '-F' & (for example)
I will see run select on pg_database all database or not and can conntect (\c
command from pgsql) to database from PGDATA and PGDATA2?

In doc for 7.2.1 wrote:

======================
Unlike most system catalogs, pg_database is shared across all databases of a
cluster: there is only one copy of pg_database per cluster, not one per
database.
=======================

Thanks

Sorry for my English

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