PostgreSQL

Readers see a consistent snapshot without blocking writers, because an update creates a new row version rather than overwriting the old one.

Dead row versions left by MVCC occupy space until a background process reclaims them; if it falls behind, tables and indexes grow without the data growing.

PostgreSQL transaction IDs are 32-bit and wrap around, so rows must be frozen before the counter laps them or data would appear to vanish.

The contract for which concurrency anomalies a transaction may observe, traded against how much concurrency the engine allows.

The delay between a write committing on the primary and becoming visible on a replica.

Relevant documents are retrieved at query time and supplied as context, so the model answers from evidence rather than parametric memory.

Text or images are embedded as vectors and compared by distance, so results are ranked by similarity rather than keyword match.

One logical write causes several physical writes - to the log, the data pages, and every affected index.

The structure that defines how data is organised in a database: tables, columns, data types, constraints, relationships, and indexes.

Organising tables so that each fact is stored once, reducing redundancy at the cost of more joins.

A data structure maintained alongside a table that makes lookups by specific columns faster, at the cost of slower writes and additional storage.

A named block of SQL (or procedural code) stored in the database and executed on the server, avoiding the round-trip cost of sending each statement from the application.

A column or set of columns that uniquely identifies every row in a table, enforced by a unique index and a NOT NULL constraint.

A constraint that links a column in one table to the primary key of another, enforcing referential integrity so that orphan rows cannot exist.

A sequence of operations that either all succeed (commit) or all fail (rollback), guaranteeing that the database moves from one consistent state to another.

Code that the database executes automatically when a specific event occurs on a table - typically an INSERT, UPDATE, or DELETE.

A named query stored in the database that behaves like a virtual table - it does not hold data itself but runs its defining query each time it is read.