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
[ 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.
The isolation level of a transaction determines what data the transaction can see when other transactions are running concurrently:
A statement can only see rows committed before it began. This is the default.
All statements of the current transaction can only see rows committed before the first query or data-modification statement was executed in this transaction.
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 SQLSTATE.
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 of these properties are set on 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.
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, 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 for more information.
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.
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 DEFERRABLE transaction_mode is a PostgreSQL language extension.
The SQL standard requires commas between successive transaction_modes, but for historical reasons PostgreSQL allows the commas to be omitted.