Determine whether the running PostgreSQL cluster is the primary one or not.

pg_get_cluster_role.sql

SELECT DISTINCT
    CASE
        WHEN b.sender=0 AND c.receiver=0 THEN
            'standalone'
        WHEN b.sender>0 AND c.receiver=0 THEN
            'primary'
        WHEN b.sender=0 AND c.receiver>0 THEN
            'replica'
        WHEN b.sender>0 AND c.receiver>0 THEN
            'primary+replica'
    END AS pg_cluster_role
FROM
    pg_database a,
    (
        SELECT count(*) AS sender
        FROM pg_stat_replication
    ) b,
    (
        SELECT count(*) AS receiver
        FROM pg_stat_wal_receiver
    ) c
WHERE
    NOT a.datistemplate;