11. Understanding JDBC Connectivity : JDBC Implementation Considerations : BLOB Column Handling : LOB Data Streams
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LOB Data Streams
A LOB data stream value accompanies the other values in a set of parameters or columns. LOB data streams are serialized in order with the other values they accompany and must be processed entirely before accessing the values that follow. LOB data streams can be accessed only once per value. The driver declares LOB values, when represented as data streams, to be of type LONGVARBINARY or LONGVARCHAR.
A LOB data stream must be accessed and read completely prior to accessing any value that follows the LOB in a result set. When a value is accessed that follows an unaccessed or partially accessed LOB data stream, the driver must read and discard the remaining LOB data so that the requested value can be accessed. If an attempt is subsequently made to access the discarded LOB value, an SQLException is generated indicating that the LOB data is no longer accessible.
LOB data streams must also be read fully before making any further request on the associated connection. Because data from the DBMS Server is serialized on the connection, the results from additional requests on the connection are queued behind any unread LOB data. The JDBC Driver avoids conflicts resulting from multiple simultaneous requests on a connection by locking the connection for the duration of each request.
When a LOB data stream value is present in a result set, the connection is not unlocked until all the data in the row, including the LOB data, has been read. An attempt to make an additional request on a connection when a LOB column has not been read completely generates an SQLException indicating that a request was made before the prior request had completed.
LOB data streams can be accessed only once. Because LOB data streams are not cached, only one call (to getString(), getCharacterStream(), and so on) can be made for each LOB value in each row of the result set. Additional requests to access a LOB data stream value generate an SQLException indicating that the LOB data is no longer available.
In general, the following recommendation from the Sun JDBC documentation must be followed: “For maximum portability, columns within a row must be read in left-to-right order, and each column must only be read once. This reflects implementation limitations in some underlying database protocols.”
A result set containing a LOB data stream value is not able to perform READONLY cursor pre-fetching. Only one row of a result set is retrieved with each DBMS Server access when a LOB data stream is present. While this does not directly affect the JDBC application, row fetch performance is reduced when a result set contains a LOB data stream value.
The restrictions associated with LOB data streams can be avoided by configuring the driver to cache LOB data streams when received from the DBMS. When enabled, the driver reads LOB data streams as they are received and stores them in memory. All row column values are fully loaded when control is returned to the application.
An uncached LOB data stream can also be cached by accessing it using the getBlob() or getClob() method. Calling one of these methods satisfies the restrictions associated with LOB data streams and allows extended access to the LOB data using the Blob/Clob interface.
For further details on caching LOB data streams, see Cached LOB Values below.