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 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 9.82
   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.