C Coding Tips

Disabling Auto-Commit Mode

Auto-commit mode indicates to the database whether to issue an automatic COMMIT operation after every SQL operation. Being in auto-commit mode can be expensive in terms of time and processing effort if, for example, you are repeating the same statement with different bind variables.

By default, new connection objects are in auto-commit mode. However, you can disable auto-commit mode with the setAutoCommit method of the connection object, either java.sql.Conection or oracle.jdbc.OracleConnection.

In auto-commit mode, the COMMIT operation occurs either when the statement completes or the next execute occurs, whichever comes first. In the case of statements returning a ResultSet object, the statement completes when the last row of the Result Set has been retrieved or when the Result Set has been closed. In more complex cases, a single statement can return multiple results as well as output parameter values. Here, the COMMIT occurs when all results and output parameter values have been retrieved.

If you disable auto-commit mode with a setAutoCommit(false) call, then you must manually commit or roll back groups of operations using the commit or rollback method of the connection object.

Example

The following example illustrates loading the driver and connecting to the database. Because new connections are in auto-commit mode by default, this example shows how to disable auto-commit. In the example, conn represents the Connection object, and stmt represents the Statement object.

// Connect to the database 
// You can put a database host name after the @ sign in the connection URL.
   OracleDataSource ods = new OracleDataSource();
   ods.setURL("jdbc:oracle:oci:@");
   ods.setUser("scott");
   ods.setPassword("tiger");
   Connection conn = ods.getConnection();
    
// It's faster when auto commit is off 
conn.setAutoCommit (false);

// Create a Statement 
Statement stmt = conn.createStatement (); 
...

Standard Fetch Size and Oracle Row Prefetching

Oracle JDBC connection and statement objects allow you to specify the number of rows to prefetch into the client with each trip to the database while a result set is being populated during a query. You can set a value in a connection object that affects each statement produced through that connection, and you can override that value in any particular statement object. The default value in a connection object is 10. Prefetching data into the client reduces the number of round-trips to the server.

Similarly, and with more flexibility, JDBC 2.0 enables you to specify the number of rows to fetch with each trip, both for statement objects (affecting subsequent queries) and for result set objects (affecting row refetches). By default, a result set uses the value for the statement object that produced it. If you do not set the JDBC 2.0 fetch size, then the Oracle connection row-prefetch value is used by default.

Standard and Oracle Update Batching

Oracle JDBC drivers allow you to accumulate INSERT, DELETE, and UPDATE operations of prepared statements at the client and send them to the server in batches. This feature reduces round-trips to the server. You can either use Oracle update batching, which typically processes a batch implicitly once a preset batch value is reached, or standard update batching, where the batch is processed explicitly.

Statement Caching

Statement caching improves performance by caching executable statements that are used repeatedly, such as in a loop or in a method that is called repeatedly. Applications use the statement cache to cache statements associated with a particular physical connection. When you enable Statement caching, a Statement object is cached when you call the close method. Because each physical connection has its own cache, multiple caches can exist if you enable Statement caching for multiple physical connections.

When you enable Statement caching on a connection cache, the logical connections benefit from the Statement caching that is enabled on the underlying physical connection. If you try to enable Statement caching on a logical connection held by a connection cache, then this will throw an exception.

Mapping Between Built-in SQL and Java Types

The SQL built-in types are those types with system-defined names, such as NUMBER, and CHAR, as opposed to the Oracle objects, varray, and nested table types, which have user-defined names. In JDBC programs that access data of built-in SQL types, all type conversions are unambiguous, because the program context determines the Java type to which a SQL datum will be converted.

The most efficient way to access numeric data is to use primitive Java types like int, float, long, and double. However, the range of values of these types do not exactly match the range of values of the SQL NUMBER data type. As a result, there may be some loss of information. If absolute precision is required across the entire value range, then use the BigDecimal type.

All character data is converted to the UCS2 character set of Java. The most efficient way to access character data is as java.lang.String. In worst case, this can cause a loss of information when two or more characters in the database character set map to a single UCS2 character. In Oracle Database 11g, all characters in the character set map to the characters in the UCS2 character set. However, some characters do map to surrogate pairs.