I wanna get how many queries and transactions my DB is doing per second...
is that a way to get that? maybe using pg_stat_statements?
As mentioned in another reply, you can determine the queries per second being executed if you turn on full logging and run the log through pgbadger (for example).
For transactions, sampling the pg_stat_database relation will give you transaction stats.
For example: SELECT current_setting('port')::INTEGER as port, datname AS dbname, SUM(xact_commit)+SUM(xact_rollback) AS transactions FROM pg_stat_database WHERE datname NOT IN ('template0','template1') GROUP BY datname ;