Comparing SET TRANSACTION with SET ISOLATION

The SET TRANSACTION statement complies with ANSI SQL-92. This statement is similar to the Informix® SET ISOLATION statement; however, the SET ISOLATION statement is not ANSI compliant and does not provide access modes. In fact, the isolation levels that you can set with the SET TRANSACTION statement are almost parallel to the isolation levels that you can set with the SET ISOLATION statement, as the following table shows.

SET TRANSACTION Isolation Level	SET ISOLATION Isolation Level
Read Uncommitted	Dirty Read
Read Committed	Committed Read
[ Not supported ]	Cursor Stability
(ANSI) Repeatable Read	(Informix) Repeatable Read
Serializable	(Informix) Repeatable Read

Another difference between SET TRANSACTION and SET ISOLATION is the behavior of the isolation levels within transactions. You can issue SET TRANSACTION only once for a transaction. Any cursors that are opened during that transaction are guaranteed that isolation level (or access mode, if you are defining an access mode). With SET ISOLATION, after a transaction is started, you can change the isolation level more than once within the transaction.

The following examples illustrate this difference in the behavior of the SET ISOLATION and SET TRANSACTION statements:

EXEC SQL BEGIN WORK;
EXEC SQL SET ISOLATION TO DIRTY READ;
EXEC SQL SELECT ... ;
EXEC SQL SET ISOLATION TO REPEATABLE READ;
EXEC SQL INSERT ... ;
EXEC SQL COMMIT WORK;    -- Executes without error

Compare the previous example to these SET TRANSACTION statements:

EXEC SQL BEGIN WORK;
EXEC SQL SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
EXEC SQL SELECT ... ;
EXEC SQL SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
   -- Produces error 876: Cannot issue SET TRANSACTION
   -- in an active transaction.

An additional difference between SET ISOLATION and SET TRANSACTION is the duration of isolation levels. Because SET ISOLATION supports complete-connection level settings, the isolation level specified by SET ISOLATION remains in effect until another SET ISOLATION statement is issued. The isolation level set by SET TRANSACTION only remains in effect until the transaction terminates. Then the isolation level is reset to the default for the database type.