PostgreSQL Functionality (cont)
Uses multi-version concurrency control (MVCC)
- multiple "versions" of the database exist together
- a transaction sees the version that was valid at its start-time
- readers don't block writers; writers don't block readers
- this significantly reduces the need for locking
Disadvantages of this approach:
- extra storage for old versions of tuples (
vacuum fixes this)
- need to check "visibility" of every tuple fetched
PostgreSQL also provides locking to enforce critical concurrency.
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