PostgreSQL 8.4.2 Documentation
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# SET TRANSACTION

## 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 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.

## 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 Chapter 18 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 Chapter 13 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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