11. Understanding JDBC Connectivity : JDBC Implementation Considerations : BLOB Column Handling : Cached LOB Values
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Cached LOB Values
The JDBC Driver will cache a LOB value in three circumstances:
Caching of LOB data streams has been enabled with the system property ingres.jdbc.lob.cache.enabled.
The application calls getBlob() or getClob() for a LONGVARBINARY or LONGVARCHAR column.
The application calls a Blob/Clob modification method on an object representing a LOB Locator.
The driver can be configured to automatically cache LOB data streams by setting the following system property:
Automatically caching LOB values requires sufficient memory to hold all active LOB values. Memory resources may be severely impacted when caching is enabled. To reduce the impact of extremely large LOB values, the LOB cache stores values as a series of blocks or segments. The default size of a segment is 8192 bytes or characters. The segment size is a trade off between the number of segments needed to store a value, the size of memory blocks needed to hold a segment, and the amount of unused space in the last segment. The segment size can be configured using the following system property:
The driver provides compatibility between the LOB data stream and Locator representations by allowing the same getXXX() method calls for both types. The driver supports getBlob() and getClob() methods for LOB data streams by caching the LOB data in the Blob/Clob object.
The driver supports Blob/Clob methods that write or truncate LOB values by reading the LOB data from the database (if necessary) and caching the data in the Blob/Clob object. Modify operations therefore modify a copy of the LOB data stored in the driver. The modified data is not automatically propagated to the database. An application can write modified LOB data back to the database by updating the row holding the LOB and providing the Blob/Clob object holding the modified data as a parameter using the setBlob(), setClob(), updateBlob(), or updateClob() methods.