In PostgreSQL every row has two numbers. The transaction ID it was insert in and the transaction ID it was deleted in. An update is an insert plus a delete. When running a select in PostgreSQL you just traverse the table and for each row check these two numbers to know if you are allowed to see the row.
The details above are PostgreSQL specific but most other databases have the same problem with there being no way to know the exact count without actually counting the rows.
1. There is contention currently in PostgreSQL when writing the database journal (used for crash recovery and replication).
2. There are some optimization which are done here. For example HOT to avoid index updates.