SET TRANSACTION(7)	PostgreSQL 9.2.24 Documentation	    SET TRANSACTION(7)



NAME
       SET_TRANSACTION - set the characteristics of the current transaction

SYNOPSIS
       SET TRANSACTION transaction_mode [, ...]
       SET TRANSACTION SNAPSHOT snapshot_id
       SET SESSION CHARACTERISTICS AS TRANSACTION transaction_mode [, ...]

       where transaction_mode is one of:

	   ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
	   READ WRITE | READ ONLY
	   [ NOT ] DEFERRABLE

DESCRIPTION
       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. In addition, a snapshot can be selected, though only
       for the current transaction, not as a session default.

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

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

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

       SERIALIZABLE
	   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 error.
       The SQL standard defines one additional level, READ UNCOMMITTED. In
       PostgreSQLREAD 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 three of these
       properties are selected for 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.

       The SET TRANSACTION SNAPSHOT command allows a new transaction to run
       with the same snapshot as an existing transaction. The pre-existing
       transaction must have exported its snapshot with the pg_export_snapshot
       function (see Section 9.26.5, "Snapshot Synchronization Functions", in
       the documentation). That function returns a snapshot identifier, which
       must be given to SET TRANSACTION SNAPSHOT to specify which snapshot is
       to be imported. The identifier must be written as a string literal in
       this command, for example '000003A1-1'.	SET TRANSACTION SNAPSHOT can
       only be executed at the start of a transaction, before the first query
       or data-modification statement (SELECT, INSERT, DELETE, UPDATE, FETCH,
       or COPY) of the transaction. Furthermore, the transaction must already
       be set to SERIALIZABLE or REPEATABLE READ isolation level (otherwise,
       the snapshot would be discarded immediately, since READ COMMITTED mode
       takes a new snapshot for each command). If the importing transaction
       uses SERIALIZABLE isolation level, then the transaction that exported
       the snapshot must also use that isolation level. Also, a non-read-only
       serializable transaction cannot import a snapshot from a read-only
       transaction.

NOTES
       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. But that
       option is not available for SET TRANSACTION SNAPSHOT.

       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.

EXAMPLES
       To begin a new transaction with the same snapshot as an already
       existing transaction, first export the snapshot from the existing
       transaction. That will return the snapshot identifier, for example:

	   BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
	   SELECT pg_export_snapshot();
	    pg_export_snapshot
	   --------------------
	    000003A1-1
	   (1 row)

       Then give the snapshot identifier in a SET TRANSACTION SNAPSHOT command
       at the beginning of the newly opened transaction:

	   BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
	   SET TRANSACTION SNAPSHOT '000003A1-1';

COMPATIBILITY
       These commands are defined in the SQL standard, except for the
       DEFERRABLE transaction mode and the SET TRANSACTION SNAPSHOT form,
       which are PostgreSQL extensions.

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



PostgreSQL 9.2.24		  2017-11-06		    SET TRANSACTION(7)