Oracle’s DBMS_LOB package provides a comprehensive collection of procedures and functions for managing large objects (LOBs), such as BLOBs (Binary Large OBjects), CLOBs (Character Large OBjects), and NCLOBs (National Character Large OBjects). The WRITE procedure within the DBMS_LOB package is particularly useful for manipulating LOB values directly within the database.
Overview of DBMS_LOB.WRITE
The DBMS_LOB.WRITE procedure writes data to a LOB, starting at a specified offset. It allows for modifying LOB values by overwriting existing data or adding new data within a LOB. This procedure is critical for applications that need to handle large amounts of data, such as document management systems, multimedia libraries, or any application that stores large data objects.
The basic syntax of the DBMS_LOB.WRITE procedure is as follows:
DBMS_LOB.WRITE( lob_loc IN OUT NOCOPY CLOB | BLOB, amount IN OUT INTEGER, offset IN INTEGER, buffer IN RAW | VARCHAR2);
lob_loc: A LOB locator pointing to the LOB to be written. It must be a locator that was obtained from a LOB column in a row of a table. The IN OUT NOCOPY parameter mode improves performance by passing the locator by reference rather than by value.
amount: The number of bytes (for BLOBs) or characters (for CLOBs or NCLOBs) to write. This parameter is IN OUT, meaning the procedure updates it to reflect the actual number of bytes or characters written.
offset: The position in the LOB at which to begin writing. The first position in the LOB is 1.
buffer: The data to be written into the LOB. For BLOBs, this is RAW data; for CLOBs and NCLOBs, this is character data (VARCHAR2).
To write to a CLOB:
DECLARE lob_loc CLOB; amt INTEGER := 10; -- Amount of characters to write offset INTEGER := 1; -- Start at the beginning BEGIN SELECT my_clob INTO lob_loc FROM my_table WHERE my_id = 1 FOR UPDATE; DBMS_LOB.WRITE(lob_loc, amt, offset, 'Hello World'); -- Updates the lob_loc with 'Hello World', overwriting the first 10 characters. END;
When using DBMS_LOB.WRITE, ensure that the LOB is selected FOR UPDATE to lock the row containing the LOB.
The maximum size of data that can be written in a single operation depends on the database version and the data type of the LOB.
Remember to commit the transaction to make changes permanent.
DBMS_LOB.WRITE is a powerful tool for working with LOBs in Oracle, allowing for efficient, programmatic manipulation of large data objects directly within the database. Its ability to modify LOBs at specific offsets provides the flexibility needed for developing applications that manage large datasets.