Choosing the appropriate type of cursor allows maximum application flexibility. This section summarizes the performance issues of three types of cursors: forward-only, insensitive, and sensitive.
A forward-only cursor provides excellent performance for sequential reads of all rows in a table. For retrieving table data, there is no faster way to return result rows than using a forward-only cursor; however, forward-only cursors cannot be used when the rows to be returned are not sequential.
Insensitive cursors are ideal for applications that require high levels of concurrency on the database server and require the ability to scroll forwards and backwards through result sets. The first request to an insensitive cursor fetches all the rows and stores them on the client. In most cases, the first request to an insensitive cursor fetches all the rows and stores them on the client. If a driver uses "lazy" fetching (fetch-on-demand), the first request may include many rows, if not all rows.The initial request is slow, especially when long data is returned. Subsequent requests do not require any network traffic (or, when a driver uses "lazy" fetching, requires limited network traffic) and are processed quickly.
Because the first request is processed slowly, insensitive cursors should not be used for a single request of one row. Developers should also avoid using insensitive cursors when long data or large result sets are returned because memory can be exhausted. Some insensitive cursor implementations cache the data in a temporary table on the database server and avoid the performance issue, but most cache the information local to the application.
Sensitive cursors, or keyset-driven cursors, use identifiers such as a ROWID that already exist in the database. When you scroll through the result set, the data for these identifiers is returned. Because each request generates network traffic, performance can be very slow. However, returning non-sequential rows does not further affect performance.
To illustrate this point further, consider an application that normally returns 1000 rows to an application. At execute time, or when the first row is requested, a JDBC driver does not execute the Select statement that was provided by the application. Instead, the JDBC driver replaces the Select list of the query with a key identifier, for example, ROWID. This modified query is then executed by the driver and all 1000 key values are returned by the database server and cached for use by the driver. Each request from the application for a result row directs the JDBC driver to look up the key value for the appropriate row in its local cache, construct an optimized query that contains a Where clause similar to WHERE ROWID=?, execute the modified query, and return the single result row from the server.
Sensitive cursors are the preferred scrollable cursor model for dynamic situations when the application cannot afford to buffer the data associated with an insensitive cursor.
