POSTGRES - Quais os bancos que ocupam mais espaço em disco?

SELECT d.datname AS Name,  pg_catalog.pg_get_userbyid(d.datdba) AS Owner,
    CASE WHEN pg_catalog.has_database_privilege(d.datname, 'CONNECT')
        THEN pg_catalog.pg_size_pretty(pg_catalog.pg_database_size(d.datname))
        ELSE 'No Access'
    END AS Size
FROM pg_catalog.pg_database d
    ORDER BY
    CASE WHEN pg_catalog.has_database_privilege(d.datname, 'CONNECT')
        THEN pg_catalog.pg_database_size(d.datname)
        ELSE NULL
    END DESC -- nulls first
    LIMIT 20

Fonte: https://wiki.postgresql.org/wiki/Disk_Usage 

POSTGRES - Quais os SQLs que estão demorando mais tempo para serem executados neste momento?

SELECT datname, usename, now()-query_start as tempo_execucao, waiting, client_addr, current_query FROM pg_stat_activity WHERE current_query <> ''order by 3 desc;


Versões de Postgres 9:
SELECT datname, usename, now()-query_start as tempo_execucao, waiting, client_addr, query FROM pg_stat_activity WHERE query <> ''order by 3 desc;