66.1. 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'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
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
.