Advanced Transaction Control

SQLJ supports the SQL SET TRANSACTION statement to specify the access mode and isolation level of any given transaction. Supported settings for access mode are READ ONLY and READ WRITE. Supported settings for isolation level are SERIALIZABLE, READ COMMITTED, READ UNCOMMITTED, and REPEATABLE READ. Oracle SQL, however, does not support READ UNCOMMITTED or REPEATABLE READ.

READ WRITE is the default access mode in both standard SQL and Oracle SQL.

READ COMMITTED is the default isolation level in Oracle SQL; SERIALIZABLE is the default in standard SQL.

Access modes and isolation levels are briefly described below. For more information, see the Oracle8i SQL Reference. You might also consult any guide to standard SQL for additional conceptual information.

For an overview of transactions and information about SQLJ support for more basic transaction control functions, such as COMMIT and ROLLBACK, see "Basic Transaction Control".

SET TRANSACTION Syntax

In SQLJ, the SET TRANSACTION statement has the following syntax:

#sql { SET TRANSACTION <access_mode>, <ISOLATION LEVEL isolation_level> };

If you use SET TRANSACTION it must be the first statement in your transaction (in other words, the first statement since your connection to the database or your most recent COMMIT or ROLLBACK), preceding any DML statements.

In a SET TRANSACTION statement, you can optionally specify the isolation level first, or specify only the access mode or only the isolation level. Following are some examples:

#sql { SET TRANSACTION READ ONLY };

#sql { SET TRANSACTION ISOLATION LEVEL SERIALIZABLE };

#sql { SET TRANSACTION READ WRITE, ISOLATION LEVEL SERIALIZABLE };

#sql { SET TRANSACTION ISOLATION LEVEL READ COMMITTED, READ ONLY };

Note that in SQLJ both the access mode and the isolation level can be set in a single SET TRANSACTION statement. This is not true in other Oracle SQL tools such as Server Manager or SQL*Plus, where a single statement can set one or the other but not both.

Access Mode Settings

The READ WRITE and READ ONLY access mode settings have the following functionality:

• READ WRITE (default)--In a READ WRITE transaction, the user is allowed to update the database. SELECT, INSERT, UPDATE, and DELETE are all legal.

• READ ONLY--In a READ ONLY transaction, the user is not allowed to update the database. SELECT is legal, but INSERT, UPDATE, DELETE, and SELECT FOR UPDATE are not.

Isolation Level Settings

The READ COMMITTED, SERIALIZABLE, READ UNCOMMITTED, and REPEATABLE READ isolation level settings (where supported) have the following functionality:

• READ UNCOMMITTED (not supported by Oracle8i)--Dirty reads, non-repeatable reads, and phantom reads are all allowed.

• READ COMMITTED--Dirty reads are prevented; non-repeatable reads and phantom reads are allowed. If the transaction contains DML statements that require row locks held by other transactions, then any of the statements will block until the row lock it needs is released by the other transaction. (See below for definitions of the italicized terms.)

• REPEATABLE READ (not supported by Oracle8i)--Dirty reads and non-repeatable reads are prevented; phantom reads are allowed.

• SERIALIZABLE--Dirty reads, non-repeatable reads, and phantom reads are all prevented. Any DML statements in the transaction cannot update any resource that may have had changes committed after the transaction began. Such DML statements will fail.

A dirty read occurs when transaction B accesses a row that was updated by transaction A, but transaction A later rolls back the updates. As a result, transaction B sees data that was never actually committed to the database.

A non-repeatable read occurs when transaction A retrieves a row, transaction B subsequently updates the row, and transaction A later retrieves the same row again. Transaction A retrieves the same row twice but sees different data.

A phantom read occurs when transaction A retrieves a set of rows satisfying a given condition, transaction B subsequently inserts or updates a row such that the row now meets the condition in transaction A, and transaction A later repeats the conditional retrieval. Transaction A now sees an additional row; this row is referred to as a "phantom".

You can think of the four isolation level settings being in a progression:

SERIALIZABLE > REPEATABLE READ > READ COMMITTED > READ UNCOMMITTED

If a desired setting is unavailable to you--such as REPEATABLE READ or READ UNCOMMITTED if you use an Oracle database--use a "greater" setting (one further to the left) to ensure having at least the level of isolation that you want.

Using JDBC Connection Class Methods

You can optionally access and set the access mode and isolation level of a transaction using methods of the underlying java.sql.Connection instance of your connection context instance.

Note that this is not recommended. SQLJ code using these JDBC methods is not portable.

Following are the Connection class methods for access mode and isolation level settings:

• public abstract int getTransactionIsolation()--Returns the current transaction isolation level as one of the following constant values:
  TRANSACTION_NONE
TRANSACTION_READ_COMMITTED
TRANSACTION_SERIALIZABLE
TRANSACTION_READ_UNCOMMITTED
TRANSACTION_REPEATABLE_READ

• public abstract void setTransactionIsolation(int)--Sets the transaction isolation level, taking as input one of the preceding constant values.

• public abstract boolean isReadOnly()--Returns true if the transaction is READ ONLY; returns false if the transaction is READ WRITE.

• public abstract void setReadOnly(boolean)--Sets the transaction access mode to READ ONLY if true is input; sets the access mode to READ WRITE if false is input.

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