Oracle’s DBMS_LOB.READ procedure is a part of the DBMS_LOB package, which provides a collection of procedures and functions for managing Large Objects (LOBs). LOBs in Oracle database are used to store large amounts of data, including text, images, videos, and other multimedia formats. There are different types of LOBs, such as BLOB (Binary Large Object), CLOB (Character Large Object), NCLOB (National Character Large Object), and BFILE (Binary File).
The DBMS_LOB.READ procedure is specifically designed to read data from a LOB value. It allows applications to read parts of a LOB or the entire LOB data, depending on the requirements. This procedure is particularly useful when dealing with large datasets that cannot be efficiently handled using regular data types or when the data exceeds the size limits of VARCHAR2 or RAW types.
Syntax
The basic syntax of the DBMS_LOB.READ procedure is as follows:
DBMS_LOB.READ ( lob_loc IN BLOB, amount IN OUT NOCOPY NUMBER, offset IN NUMBER, buffer OUT RAW);
lob_loc: The LOB locator that points to the LOB you want to read. This can be a BLOB, CLOB, NCLOB, or BFILE.
amount: Specifies the number of characters (for CLOB or NCLOB) or bytes (for BLOB or BFILE) to be read. This parameter is both IN and OUT. It acts as an input to specify how much data to read and as an output to indicate how much was actually read.
offset: The offset from the beginning of the LOB at which to start reading. The first position in the LOB is 1.
buffer: The output parameter that stores the data read from the LOB. For CLOBs or NCLOBs, the data type of the buffer would be VARCHAR2; for BLOBs or BFILEs, it would be RAW.
Usage
DBMS_LOB.READ is typically used in PL/SQL blocks, procedures, and functions where manipulating large data is necessary. It allows for efficiently reading large or small portions of data stored in LOBs.
Example
Here is a simple example that demonstrates how to read data from a CLOB:
DECLARE my_clob CLOB; buffer VARCHAR2(32767); amount NUMBER := 32767; -- Amount of data to read offset NUMBER := 1; -- Starting position BEGIN SELECT my_clob_column INTO my_clob FROM my_table WHERE my_condition; DBMS_LOB.READ(my_clob, amount, offset, buffer); -- Process the buffer content here DBMS_OUTPUT.PUT_LINE(buffer); END;
In this example, the DBMS_LOB.READ procedure is used to read up to 32,767 characters from a CLOB stored in my_table. The data is read starting from the first character and stored in the buffer variable, which is then output using DBMS_OUTPUT.PUT_LINE.
Considerations
When using DBMS_LOB.READ, it’s important to handle exceptions, especially NO_DATA_FOUND, which is raised if the offset is beyond the end of the LOB.
For performance reasons, consider the size of the chunks you’re reading, especially in a loop, to balance between the number of reads and the amount of memory used.
DBMS_LOB.READ is a powerful tool for working with large objects in Oracle databases, enabling fine-grained access and manipulation of large data sets.