SET TRANSACTION SQL - Language Statements SET TRANSACTION Set the characteristics of the current SQL-transaction 2000-06-01 SET TRANSACTION ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } 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 DML statement (SELECT, INSERT, DELETE, UPDATE, FETCH, COPY) of a transaction has been executed. 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 DML statement was executed in this transaction. 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. SERIALIZABLE is the default level in SQL. Postgres does not provide the isolation levels READ UNCOMMITTED and REPEATABLE READ. Because of multi-version 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 this command: whether the transaction is read-only and the size of the diagnostics area. Neither of these concepts are supported in Postgres.