Provided by: postgresql-client-9.1_9.1.3-2_i386 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.