SET TRANSACTION — set the characteristics of the current transaction
SET TRANSACTIONtransaction_mode[, ...] SET TRANSACTION SNAPSHOTsnapshot_idSET SESSION CHARACTERISTICS AS TRANSACTIONtransaction_mode[, ...] wheretransaction_modeis one of: ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED } READ WRITE | READ ONLY [ NOT ] DEFERRABLE
   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, the transaction access mode (read/write or read-only), and the deferrable mode. In addition, a snapshot can be selected, though only for the current transaction, not as a session default.
The isolation level of a transaction determines what data the transaction can see when other transactions are running concurrently:
READ COMMITTEDA statement can only see rows committed before it began. This is the default.
REPEATABLE READAll statements of the current transaction can only see rows committed before the first query or data-modification statement was executed in this transaction.
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.  If a pattern of reads and writes among concurrent
       serializable transactions would create a situation which could not
       have occurred for any serial (one-at-a-time) execution of those
       transactions, one of them will be rolled back with a
       serialization_failure error.
      
   The SQL standard defines one additional level, READ
   UNCOMMITTED.
   In PostgreSQL READ
   UNCOMMITTED is treated as READ COMMITTED.
  
   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
   Chapter 13 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.
  
   The DEFERRABLE transaction property has no effect
   unless the transaction is also SERIALIZABLE and
   READ ONLY.  When all three of these properties are
   selected for a
   transaction, the transaction may block when first acquiring its snapshot,
   after which it is able to run without the normal overhead of a
   SERIALIZABLE transaction and without any risk of
   contributing to or being canceled by a serialization failure.  This mode
   is well suited for long-running reports or backups.
  
   The SET TRANSACTION SNAPSHOT command allows a new
   transaction to run with the same snapshot as an existing
   transaction.  The pre-existing transaction must have exported its snapshot
   with the pg_export_snapshot function (see Section 9.26.5).  That function returns a
   snapshot identifier, which must be given to SET TRANSACTION
   SNAPSHOT to specify which snapshot is to be imported.  The
   identifier must be written as a string literal in this command, for example
   '00000003-0000001B-1'.
   SET TRANSACTION SNAPSHOT can only be executed at the
   start of a transaction, before the first query or
   data-modification statement (SELECT,
   INSERT, DELETE,
   UPDATE, FETCH, or
   COPY) of the transaction.  Furthermore, the transaction
   must already be set to SERIALIZABLE or
   REPEATABLE READ isolation level (otherwise, the snapshot
   would be discarded immediately, since READ COMMITTED mode takes
   a new snapshot for each command).  If the importing transaction uses
   SERIALIZABLE isolation level, then the transaction that
   exported the snapshot must also use that isolation level.  Also, a
   non-read-only serializable transaction cannot import a snapshot from a
   read-only transaction.
  
   If SET TRANSACTION is executed without a prior
   START TRANSACTION or BEGIN,
   it emits a warning and otherwise has no effect.
  
   It is possible to dispense with SET TRANSACTION
   by instead specifying the desired transaction_modes in
   BEGIN or START TRANSACTION.
   But that option is not available for SET TRANSACTION
   SNAPSHOT.
  
   The session default transaction modes can also be set or examined via the
   configuration parameters default_transaction_isolation,
   default_transaction_read_only, and
   default_transaction_deferrable.
   (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 Chapter 19
   for more information.
  
   The current transaction's modes can similarly be set or examined via the
   configuration parameters transaction_isolation,
   transaction_read_only, and
   transaction_deferrable.  Setting one of these
   parameters acts the same as the corresponding SET
   TRANSACTION option, with the same restrictions on when it can
   be done.  However, these parameters cannot be set in the configuration
   file, or from any source other than live SQL.
  
To begin a new transaction with the same snapshot as an already existing transaction, first export the snapshot from the existing transaction. That will return the snapshot identifier, for example:
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ; SELECT pg_export_snapshot(); pg_export_snapshot --------------------- 00000003-0000001B-1 (1 row)
   Then give the snapshot identifier in a SET TRANSACTION
   SNAPSHOT command at the beginning of the newly opened
   transaction:
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ; SET TRANSACTION SNAPSHOT '00000003-0000001B-1';
   These commands are defined in the SQL standard,
   except for the DEFERRABLE transaction mode
   and the SET TRANSACTION SNAPSHOT form, which are
   PostgreSQL extensions.
  
   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.
  
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.