SET TRANSACTION(7)                                        SQL Commands                                        SET TRANSACTION(7)



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 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 and the transaction access mode (read/write
       or read-only).

       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.

       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.

       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 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, TRUN-
       CATE; 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_isola-
       tion 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. Con-
       sult in 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 mentioned above. Because of lack of
       predicate locking, the SERIALIZABLE level is not truly serializable. See in 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                                  2011-09-22                                         SET TRANSACTION(7)