WITH timeset AS (
select pg_database_size(datname) num, pg_size_pretty(pg_database_size(datname)) size from pg_database where datname = 'db'
UNION ALL
select pg_database_size(datname) num, pg_size_pretty(pg_database_size(datname)) size from pg_database, pg_sleep(10) where datname = 'db' 
)
SELECT num - lag(num,1) OVER (ORDER BY num) bytes_per_10_second, size  FROM timeset
;