SET TRANSACTION ISOLATION LEVEL
{READ COMMITTED | SERIALIZABLE}
SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL
{READ COMMITTED | SERIALIZABLE}
Use this keyword to specify that statements within this transaction will be able to view changes to the database that were committed before the transaction began. This is set by default.
Use this keyword to specify that statements within this transaction will be able to view all rows that were committed in the database before the transaction's first DML statement is executed.
Use the SET TRANSACTION command to set the transaction isolation level for the current transaction. This change will only affect the current transaction; all other subsequent transactions must have their isolation mode explicitly set, otherwise the default of READ COMMITTED will be used. You can only use this command before the first DML statement has been executed. A DML statment is one of SELECT, INSERT, DELETE , UPDATE, FETCH, or COPY.
To set the default transaction isolation level (as opposed to on an individual transaction basis), use SET SESSION CHARACTERISTICS and specify either READ COMMITTED or SERIALIZABLE. Issuing a SET TRANSACTION command from within a transaction can override this default setting.
When the isolation level is set to READ COMMITTED, all statements within the transaction view only the rows that were committed before the transaction was started. Setting the isolation level to SERIALIZABLE allows statements within the transaction to view changes made to the database before the first DML statement was executed within the transaction.
The following example sets the transaction isolation level to SERIALIZABLE for the current transaction.
booktown=# SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; SET VARIABLE
The next example demonstrates how to set the default transaction isolation level for the current session.
booktown=# SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL SERIALIZABLE; SET VARIABLE