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 SES-
       SION CHARACTERISTICS sets the default transaction characteristics for subsequent transactions of a session. These defaults can be  overrid-
       den 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 exe-
              cuted 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  concur-
       rency 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  tem-
       porary  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 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  omit-
       ted.

SQL - Language Statements                                           2010-05-14                                                  SET TRANSACTION(7)