Both InnoDB and PostgreSQL - as well as many other databases - use a technique called multi-version concurrency control (MVCC) to provide transaction isolation: transactions should not see the work of other, uncommitted transactions. MVCC means that, when a row is updated, the database stores both the old and new versions of the row.
(Not All written by me.... )Some time just monitoring a DB servers from monitoring tools is just not enough, in that case we need some tool that let us know, what is DB is holding, processing, parameters etc..
