# Transaction Processing

This chapter provides an overview of the internals of PostgreSQL's transaction management system. The word transaction is often abbreviated as xact.

## 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's process number (or `procNumber`) and a sequentially-assigned number local to each backend, known as `localXID`. For example, the virtual transaction ID `4/12532` has a `procNumber` 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 functions-pg-snapshot 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 guc-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.

## Transactions and Locking

The transaction IDs of currently executing transactions are shown in pg_locks in columns virtualxid and transactionid. Read-only transactions will have virtualxids but NULL transactionids, while both columns will be set in read-write transactions.

Some lock types wait on virtualxid, while other types wait on transactionid. Row-level read and write locks are recorded directly in the locked rows and can be inspected using the pgrowlocks extension. Row-level read locks might also require the assignment of multixact IDs (`mxid`; see vacuum-for-multixact-wraparound).

## Subtransactions

Subtransactions are started inside transactions, allowing large transactions to be broken into smaller units. Subtransactions can commit or abort without affecting their parent transactions, allowing parent transactions to continue. This allows errors to be handled more easily, which is a common application development pattern. The word subtransaction is often abbreviated as subxact.

Subtransactions can be started explicitly using the `SAVEPOINT` command, but can also be started in other ways, such as PL/pgSQL's `EXCEPTION` clause. PL/Python and PL/Tcl also support explicit subtransactions. Subtransactions can also be started from other subtransactions. The top-level transaction and its child subtransactions form a hierarchy or tree, which is why we refer to the main transaction as the top-level transaction.

If a subtransaction is assigned a non-virtual transaction ID, its transaction ID is referred to as a subxid. Read-only subtransactions are not assigned subxids, but once they attempt to write, they will be assigned one. This also causes all of a subxid's parents, up to and including the top-level transaction, to be assigned non-virtual transaction ids. We ensure that a parent xid is always lower than any of its child subxids.

The immediate parent xid of each subxid is recorded in the `pg_subtrans` directory. No entry is made for top-level xids since they do not have a parent, nor is an entry made for read-only subtransactions.

When a subtransaction commits, all of its committed child subtransactions with subxids will also be considered subcommitted in that transaction. When a subtransaction aborts, all of its child subtransactions will also be considered aborted.

When a top-level transaction with an xid commits, all of its subcommitted child subtransactions are also persistently recorded as committed in the `pg_xact` subdirectory. If the top-level transaction aborts, all its subtransactions are also aborted, even if they were subcommitted.

The more subtransactions each transaction keeps open (not rolled back or released), the greater the transaction management overhead. Up to 64 open subxids are cached in shared memory for each backend; after that point, the storage I/O overhead increases significantly due to additional lookups of subxid entries in `pg_subtrans`.

## Two-Phase Transactions

PostgreSQL supports a two-phase commit (2PC) protocol that allows multiple distributed systems to work together in a transactional manner. The commands are `PREPARE TRANSACTION`, `COMMIT PREPARED` and `ROLLBACK PREPARED`. Two-phase transactions are intended for use by external transaction management systems. PostgreSQL follows the features and model proposed by the X/Open XA standard, but does not implement some less often used aspects.

When the user executes `PREPARE TRANSACTION`, the only possible next commands are `COMMIT PREPARED` or `ROLLBACK PREPARED`. In general, this prepared state is intended to be of very short duration, but external availability issues might mean transactions stay in this state for an extended interval. Short-lived prepared transactions are stored only in shared memory and WAL. Transactions that span checkpoints are recorded in the `pg_twophase` directory. Transactions that are currently prepared can be inspected using pg_prepared_xacts.