PREPARE TRANSACTION

PREPARE TRANSACTION

7

SQL - Language Statements

PREPARE TRANSACTION

prepare the current transaction for two-phase commit

PREPARE TRANSACTION

transaction\_id

Description
===========

`PREPARE TRANSACTION` prepares the current transaction for two-phase
commit. After this command, the transaction is no longer associated with
the current session; instead, its state is fully stored on disk, and
there is a very high probability that it can be committed successfully,
even if a database crash occurs before the commit is requested.

Once prepared, a transaction can later be committed or rolled back with
[`COMMIT PREPARED`](#sql-commit-prepared) or
[`ROLLBACK PREPARED`](#sql-rollback-prepared), respectively. Those
commands can be issued from any session, not only the one that executed
the original transaction.

From the point of view of the issuing session, `PREPARE
   TRANSACTION` is not unlike a `ROLLBACK` command: after executing it,
there is no active current transaction, and the effects of the prepared
transaction are no longer visible. (The effects will become visible
again if the transaction is committed.)

If the `PREPARE TRANSACTION` command fails for any reason, it becomes a
`ROLLBACK`: the current transaction is canceled.

Parameters
==========

transaction\_id

:   An arbitrary identifier that later identifies this transaction for
    `COMMIT PREPARED` or `ROLLBACK PREPARED`. The identifier must be
    written as a string literal, and must be less than 200 bytes long.
    It must not be the same as the identifier used for any currently
    prepared transaction.

Notes
=====

`PREPARE TRANSACTION` is not intended for use in applications or
interactive sessions. Its purpose is to allow an external transaction
manager to perform atomic global transactions across multiple databases
or other transactional resources. Unless you\'re writing a transaction
manager, you probably shouldn\'t be using `PREPARE
   TRANSACTION`.

This command must be used inside a transaction block. Use
[`BEGIN`](#sql-begin) to start one.

It is not currently allowed to `PREPARE` a transaction that has executed
any operations involving temporary tables or the session\'s temporary
namespace, created any cursors `WITH HOLD`, or executed `LISTEN`,
`UNLISTEN`, or `NOTIFY`. Those features are too tightly tied to the
current session to be useful in a transaction to be prepared.

If the transaction modified any run-time parameters with `SET` (without
the `LOCAL` option), those effects persist after `PREPARE TRANSACTION`,
and will not be affected by any later `COMMIT PREPARED` or
`ROLLBACK PREPARED`. Thus, in this one respect `PREPARE TRANSACTION`
acts more like `COMMIT` than `ROLLBACK`.

All currently available prepared transactions are listed in the
[pg\_prepared\_xacts](#view-pg-prepared-xacts) system view.

> **Caution**
>
> It is unwise to leave transactions in the prepared state for a long
> time. This will interfere with the ability of `VACUUM` to reclaim
> storage, and in extreme cases could cause the database to shut down to
> prevent transaction ID wraparound (see [???](#vacuum-for-wraparound)).
> Keep in mind also that the transaction continues to hold whatever
> locks it held. The intended usage of the feature is that a prepared
> transaction will normally be committed or rolled back as soon as an
> external transaction manager has verified that other databases are
> also prepared to commit.
>
> If you have not set up an external transaction manager to track
> prepared transactions and ensure they get closed out promptly, it is
> best to keep the prepared-transaction feature disabled by setting
> [???](#guc-max-prepared-transactions) to zero. This will prevent
> accidental creation of prepared transactions that might then be
> forgotten and eventually cause problems.

Examples {#sql-prepare-transaction-examples}
========

Prepare the current transaction for two-phase commit, using `foobar` as
the transaction identifier:

    PREPARE TRANSACTION 'foobar';

Compatibility
=============

`PREPARE TRANSACTION` is a PostgreSQL extension. It is intended for use
by external transaction management systems, some of which are covered by
standards (such as X/Open XA), but the SQL side of those systems is not
standardized.

See Also
========
