Description
The SET TRANSACTION command sets the transaction characteristics of the current transaction. It has no effect on any subsequent transactions. SET SESSION CHARACTERISTICS sets the default transaction characteristics for each transaction of a session. SET TRANSACTION can override it 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
The current transaction can only see rows committed before first query or data-modification statement was executed in this transaction.
Tip: Intuitively, serializable means that two concurrent transactions will leave the database in the same state as if the two has been executed strictly after one another in either order.
The transaction isolation level cannot be set after the first query or data-modification statement (SELECT, INSERT, DELETE, UPDATE, FETCH, COPY) of a transaction has been executed. See Chapter 12 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 TO 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 writes to disk.
Notes
The session default transaction isolation level can also be set with the command
SET default_transaction_isolation = 'value'
and in the configuration file. Consult Section 16.4 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 described above. PostgreSQL does not provide the isolation levels READ UNCOMMITTED and REPEATABLE READ. Because of multiversion concurrency control, the SERIALIZABLE level is not truly serializable. See Chapter 12 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 only for use in embedded SQL.
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