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A deadlock is a cycle of waiting transactions: each holds a lock another needs to continue. The database detects the cycle and cancels a participant so the others can proceed. Applications must treat that cancellation as a failed transaction and decide whether to retry it.

Optimistic locking checks whether data changed before committing an update, often by comparing a version number. Pessimistic locking acquires a database lock before performing a critical change. The first avoids waiting when conflicts are rare; the second coordinates competing writers but can increase waits and deadlocks.

Isolation defines how one transaction can observe and interfere with concurrent transactions. Different levels allow different anomalies to balance correctness, concurrency and retry cost. PostgreSQL uses snapshots under MVCC, but even a stable snapshot does not automatically make every business decision safe against concurrent writes.

ACID describes four properties used to reason about reliable transactions: atomicity, consistency, isolation and durability. Together they address partial changes, preserved rules, concurrent work and persistence after commit. They do not automatically supply correct business rules or the strongest isolation level.

PostgreSQL keeps older row versions for concurrent readers, so updates and deletes can leave dead versions behind. VACUUM makes eligible dead space reusable and supports visibility and transaction-ID maintenance; ANALYZE collects statistics for planning queries. Autovacuum runs this maintenance in the background but can be limited by long-running transactions or workload settings.

A deadlock is a cycle of waiting transactions: each holds a lock another needs to continue. The database detects the cycle and cancels a participant so the others can proceed. Applications must treat that cancellation as a failed transaction and decide whether to retry it.

Optimistic locking checks whether data changed before committing an update, often by comparing a version number. Pessimistic locking acquires a database lock before performing a critical change. The first avoids waiting when conflicts are rare; the second coordinates competing writers but can increase waits and deadlocks.

Isolation defines how one transaction can observe and interfere with concurrent transactions. Different levels allow different anomalies to balance correctness, concurrency and retry cost. PostgreSQL uses snapshots under MVCC, but even a stable snapshot does not automatically make every business decision safe against concurrent writes.

ACID describes four properties used to reason about reliable transactions: atomicity, consistency, isolation and durability. Together they address partial changes, preserved rules, concurrent work and persistence after commit. They do not automatically supply correct business rules or the strongest isolation level.

PostgreSQL keeps older row versions for concurrent readers, so updates and deletes can leave dead versions behind. VACUUM makes eligible dead space reusable and supports visibility and transaction-ID maintenance; ANALYZE collects statistics for planning queries. Autovacuum runs this maintenance in the background but can be limited by long-running transactions or workload settings.

An ORM translates object-oriented data operations into database commands and maps results back into application objects. It reduces repetitive persistence code, but the database still executes real SQL with real locks, indexes and query costs. Understanding the generated query is essential when debugging correctness or performance.

SQL & PostgreSQL learning module

SQL & PostgreSQL learning module