Can you explain what the term snapshot means in the context of PostgreSQL and how it is used?

In PostgreSQL, the term "snapshot" refers to a mechanism of transaction isolation control that captures the state of the database at the moment a transaction begins.

A snapshot contains information about which transactions were active, completed, or rolled back, allowing a transaction to see a consistent set of data even if other transactions are changing the database concurrently.

This is a key element of MVCC (Multi-Version Concurrency Control), which ensures that reads do not block writes and vice versa.

For example, when you start a transaction, PostgreSQL creates a snapshot, and all queries within that transaction see the data exactly as it was at the start, even if other transactions have already modified the data.

In Python, when working with PostgreSQL through libraries (such as psycopg2), you can manage transactions, but the snapshot is created and managed automatically by the server.