NAME

       SET_TRANSACTION - set the characteristics of the current transaction

SYNOPSIS

       SET TRANSACTION transaction_mode [, ...]
       SET TRANSACTION SNAPSHOT snapshot_id
       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
           [ NOT ] DEFERRABLE

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, 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 COMMITTED
           A statement can only see rows committed before it began. This is the default.

       REPEATABLE READ
           All 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, Concurrency Control, 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, 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, “Snapshot
       Synchronization Functions”, in the documentation). 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 '000003A1-1'.  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.

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. But that option is not available for SET
       TRANSACTION SNAPSHOT.

       The session default transaction modes can also be set by setting 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 18, Server Configuration,
       in the documentation for more information.

EXAMPLES

       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
           --------------------
            000003A1-1
           (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 '000003A1-1';

COMPATIBILITY

       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.