Tell about locks and the use of transactions in PostgreSQL.

In PostgreSQL, locks are used to ensure data integrity and transaction isolation. The main types of locks:

• Row-level locks — lock individual rows in a table, allowing other transactions to work with different rows. This minimizes contention.
• Table-level locks — lock the entire table, used less frequently, for example, when changing the structure.

PostgreSQL uses MVCC (Multi-Version Concurrency Control), which allows transactions to read data without locks until a write occurs.

Transactions in PostgreSQL provide atomicity, consistency, isolation, and durability (ACID). Important:

• Start a transaction with the BEGIN command or use autocommit.
• Choose an isolation level (Read Committed, Repeatable Read, Serializable), which affects data visibility and locking.
• Monitor transaction duration to avoid long-lasting locks.

Example of using a transaction:

BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;

If a lock conflict occurs during the process, one of the transactions may be rolled back with an error, which should be handled in the application.