This command sets the transaction isolation level. The SET TRANSACTION command sets the characteristics for the current SQL-transaction. It has no effect on any subsequent transactions. This command cannot be used after the first query or data-modification statement (SELECT, INSERT, DELETE, UPDATE, FETCH, COPY) of a transaction has been executed. SET SESSION CHARACTERISTICS sets the default transaction isolation level for each transaction for a session. SET TRANSACTION can override it for an individual transaction.
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.
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 session default transaction isolation level can also be set with the command
SET default_transaction_isolation = 'value'
and in the configuration file. Consult the Administrator's Guide for more information.
SERIALIZABLE is the default level in SQL. 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 the User's Guide for details.
In SQL there are two other transaction characteristics that can be set with these commands: whether the transaction is read-only and the size of the diagnostics area. Neither of these concepts are supported in PostgreSQL.