74.1. Transactions and Identifiers
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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 `TransactionId`s (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`.

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