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, MERGE, 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, MERGE, 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, MERGE, 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.