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 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 9.27.5). 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.

NOTES

       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 20 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.

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
           ---------------------
            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';

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.