Provided by: postgresql-client-9.1_9.1.3-2_amd64 bug


       SET_TRANSACTION - set the characteristics of the current transaction


       SET TRANSACTION transaction_mode [, ...]

       where transaction_mode is one of:

           [ NOT ] DEFERRABLE


       The SET TRANSACTION command sets the characteristics of the current transaction. It has no
       effect on any subsequent transactions.  SET SESSION CHARACTERISTICS sets the default
       transaction characteristics for subsequent transactions of a session. These defaults can
       be overridden by SET TRANSACTION for an individual transaction.

       The available transaction characteristics are the transaction isolation level, the
       transaction access mode (read/write or read-only), and the deferrable mode.

       The isolation level of a transaction determines what data the transaction can see when
       other transactions are running concurrently:

           A statement can only see rows committed before it began. This is the default.

           All statements of the current transaction can only see rows committed before the first
           query or data-modification statement was executed in this transaction.

           All statements of the current transaction can only see rows committed before the first
           query or data-modification statement was executed in this transaction. If a pattern of
           reads and writes among concurrent serializable transactions would create a situation
           which could not have occurred for any serial (one-at-a-time) execution of those
           transactions, one of them will be rolled back with a serialization_failure SQLSTATE.
       The SQL standard defines one additional level, READ UNCOMMITTED. In PostgreSQL READ
       UNCOMMITTED is treated as READ COMMITTED.

       The transaction isolation level cannot be changed after the first query or
       data-modification statement (SELECT, INSERT, DELETE, UPDATE, FETCH, or COPY) of a
       transaction has been executed. See Chapter 13, Concurrency Control, in the documentation
       for more information about transaction isolation and concurrency control.

       The transaction access mode determines whether the transaction is read/write or read-only.
       Read/write is the default. When a transaction is read-only, the following SQL commands are
       disallowed: INSERT, UPDATE, DELETE, and COPY FROM if the table they would write to is not
       a temporary table; all CREATE, ALTER, and DROP commands; COMMENT, GRANT, REVOKE, TRUNCATE;
       and EXPLAIN ANALYZE and EXECUTE if the command they would execute is among those listed.
       This is a high-level notion of read-only that does not prevent all writes to disk.

       The DEFERRABLE transaction property has no effect unless the transaction is also
       SERIALIZABLE and READ ONLY. When all of these properties are set on a transaction, the
       transaction may block when first acquiring its snapshot, after which it is able to run
       without the normal overhead of a SERIALIZABLE transaction and without any risk of
       contributing to or being canceled by a serialization failure. This mode is well suited for
       long-running reports or backups.


       If SET TRANSACTION is executed without a prior START TRANSACTION or BEGIN, it will appear
       to have no effect, since the transaction will immediately end.

       It is possible to dispense with SET TRANSACTION by instead specifying the desired
       transaction_modes in BEGIN or START TRANSACTION.

       The session default transaction modes can also be set by setting the configuration
       parameters default_transaction_isolation, default_transaction_read_only, and
       default_transaction_deferrable. (In fact SET SESSION CHARACTERISTICS is just a verbose
       equivalent for setting these variables with SET.) This means the defaults can be set in
       the configuration file, via ALTER DATABASE, etc. Consult Chapter 18, Server Configuration,
       in the documentation for more information.


       Both commands are defined in the SQL standard.  SERIALIZABLE is the default transaction
       isolation level in the standard. In PostgreSQL the default is ordinarily READ COMMITTED,
       but you can change it as mentioned above.

       In the SQL standard, there is one other transaction characteristic that can be set with
       these commands: the size of the diagnostics area. This concept is specific to embedded
       SQL, and therefore is not implemented in the PostgreSQL server.

       The DEFERRABLE transaction_mode is a PostgreSQL language extension.

       The SQL standard requires commas between successive transaction_modes, but for historical
       reasons PostgreSQL allows the commas to be omitted.