Snapshots

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What is a Snapshot?

A data table contains several versions of the same row, and transactions should only show one of them at most.

Visible versions of the different rows are a snapshot.

Each transaction uses its own snapshot:

Read committed takes a snapshot at the beginning of each statement. Repeatable Read and Serializable takes a snapshot at the beginning of the first statement of a transaction.

Row Version Visibility

Tuple visibility is defined by the xmin and xmax fields of the tuple header.

Snapshot Structure

The ProcArray structure in the server contains the list of all active transactions at a point in time.

Transaction Horizon

The horizon of the transaction is the smallest xmin of the active snapshot for the transaction. This is important because any other tuples with a lower xmin aren’t required for the transaction.

The horizon for the database is the smallest xmin for all active transactions in the database. Any tuples below this can be removed, and are taken care of by the vacuuming process.

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