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SET


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 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 TO 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 mentioned 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.

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