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SET TRANSACTION()		 SQL Commands		     SET TRANSACTION()



NAME
       SET TRANSACTION - set the characteristics of the current transaction


SYNOPSIS
       SET TRANSACTION transaction_mode [, ...]
       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


DESCRIPTION
       The  SET	 TRANSACTION  command  sets the characteristics of the current
       transaction. It has no effect on any subsequent transactions. SET  SES-
       SION  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 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
	      All statements of the current transaction can only see rows com-
	      mitted before the first query or data-modification statement was
	      executed in this transaction.

       The SQL standard defines two additional levels,	READ  UNCOMMITTED  and
       REPEATABLE  READ.   In  PostgreSQL  READ UNCOMMITTED is treated as READ
       COMMITTED, while REPEATABLE READ is treated as SERIALIZABLE.

       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 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.

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.

       The  session  default  transaction modes can also be set by setting the
       configuration	 parameters	default_transaction_isolation	   and
       default_transaction_read_only.  (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 the documentation 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  men-
       tioned  above.  Because	of lack of predicate locking, the SERIALIZABLE
       level is not truly serializable. See the documentation 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 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.



SQL - Language Statements	  2010-12-14		     SET TRANSACTION()
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