Transactions, MVCC, Isolation Levels, Locking, and Consistency
Learn how PostgreSQL preserves correctness when many users and processes work with the same data concurrently.
Inside this chapter
- Why Transactions Matter
- ACID in Practice
- MVCC as a Core PostgreSQL Idea
- Transaction Example
Series navigation
Study the chapters in sequence for the clearest path from beginner PostgreSQL concepts to advanced query design and production operations. Use the navigation at the bottom of every page to move chapter by chapter.
Why Transactions Matter
Business workflows often require multiple changes that must succeed together. A payment flow may insert a transaction record, reduce inventory, and create an audit log. If one part succeeds and another fails, the system becomes inconsistent unless those steps are controlled transactionally.
ACID in Practice
| Property | Meaning | Example |
|---|---|---|
| Atomicity | All or nothing | Money transfer updates both accounts or neither |
| Consistency | Rules remain valid | Foreign keys and business rules still hold |
| Isolation | Concurrent work stays safe | Two processes do not corrupt the same inventory state |
| Durability | Committed data survives failures | Confirmed orders remain after restart |
MVCC as a Core PostgreSQL Idea
PostgreSQL uses Multi-Version Concurrency Control, or MVCC, to allow readers and writers to work concurrently with less blocking than many naive locking models. This is one of the reasons PostgreSQL behaves well under concurrent transactional load. Students aiming for advanced understanding should know that MVCC also connects to vacuuming, tuple visibility, and storage maintenance.
Transaction Example
BEGIN;
UPDATE accounts
SET balance = balance - 500
WHERE account_id = 10;
UPDATE accounts
SET balance = balance + 500
WHERE account_id = 20;
COMMIT;If something fails before commit, a rollback can undo the uncommitted changes. Advanced teams also pay attention to lock waits, deadlocks, long transactions, and isolation-level tradeoffs.