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74.1. Transactions and Identifiers #

Transactions can be created explicitly using BEGIN or START TRANSACTION and ended using COMMIT or ROLLBACK. SQL statements outside of explicit transactions automatically use single-statement transactions.

Every transaction is identified by a unique VirtualTransactionId (also called virtualXID or vxid), which is comprised of a backend ID (or backendID) and a sequentially-assigned number local to each backend, known as localXID. For example, the virtual transaction ID 4/12532 has a backendID of 4 and a localXID of 12532.

Non-virtual TransactionIds (or xid), e.g., 278394, are assigned sequentially to transactions from a global counter used by all databases within the PostgreSQL cluster. This assignment happens when a transaction first writes to the database. This means lower-numbered xids started writing before higher-numbered xids. Note that the order in which transactions perform their first database write might be different from the order in which the transactions started, particularly if the transaction started with statements that only performed database reads.

The internal transaction ID type xid is 32 bits wide and wraps around every 4 billion transactions. A 32-bit epoch is incremented during each wraparound. There is also a 64-bit type xid8 which includes this epoch and therefore does not wrap around during the life of an installation; it can be converted to xid by casting. The functions in Table 9.80 return xid8 values. Xids are used as the basis for PostgreSQL's MVCC concurrency mechanism and streaming replication.

When a top-level transaction with a (non-virtual) xid commits, it is marked as committed in the pg_xact directory. Additional information is recorded in the pg_commit_ts directory if track_commit_timestamp is enabled.

In addition to vxid and xid, prepared transactions are also assigned Global Transaction Identifiers (GID). GIDs are string literals up to 200 bytes long, which must be unique amongst other currently prepared transactions. The mapping of GID to xid is shown in pg_prepared_xacts.