set_transaction

SET TRANSACTION()		 SQL Commands		    SET TRANSACTION()



NAME
       SET TRANSACTION - set the characteristics of the current transaction


SYNOPSIS
       SET TRANSACTION
	   [ ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } ] [ READ WRITE | READ ONLY ]
       SET SESSION CHARACTERISTICS AS TRANSACTION
	   [ ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } ] [ READ WRITE | READ ONLY ]


DESCRIPTION
       The  SET	 TRANSACTION  command sets the transaction characteristics of
       the current transaction. It has no effect on any	 subsequent  transac-
       tions.  SET SESSION CHARACTERISTICS sets the default transaction char-
       acteristics for each transaction of a  session.	SET  TRANSACTION  can
       override it for an individual transaction.

       The  available  transaction characteristics are the transaction isola-
       tion level and the transaction access mode (read/write or  read-only).

       The isolation level of a transaction determines what data the transac-
       tion can see when other transactions are running concurrently.

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

       SERIALIZABLE
	      The  current  transaction	 can  only  see rows committed before
	      first query or data-modification statement was executed in this
	      transaction.

	      Tip: Intuitively, serializable means that two concurrent trans-
	      actions will leave the database in the same state as if the two
	      has been executed strictly after one another in either order.


       The transaction isolation level cannot be set after the first query or
       data-modification statement (SELECT, INSERT,  DELETE,  UPDATE,  FETCH,
       COPY)  of a transaction has been executed. See [XRef to MVCC] 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 com-
       mand they would execute is among those listed. This  is	a  high-level
       notion of read-only that does not prevent writes to disk.

NOTES
       The  session  default transaction isolation level can also be set with
       the command

       SET default_transaction_isolation = ’value’

       and in the configuration file. Consult [XRef  to	 RUNTIME-CONFIG]  for
       more information.

COMPATIBILITY
       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
       described above. PostgreSQL does not provide the isolation levels READ
       UNCOMMITTED  and	 REPEATABLE READ. Because of multiversion concurrency
       control, the SERIALIZABLE level is not truly serializable.  See	[XRef
       to MVCC] for details.

       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 only for use in embedded SQL.



SQL - Language Statements	  2008-01-03		    SET TRANSACTION()