11. Using CLOB and BLOB Data

Oracle Database uses LOB objects to store large data such as text, images, videos, and other multimedia formats. The maximum size of a LOB (large object) is limited to the size of the tablespace storing it.

There are four types of LOBs:

  • BLOB - Binary Large Object, used for storing binary data. python-oracledb uses the type oracledb.DB_TYPE_BLOB.

  • CLOB - Character Large Object, used for string strings in the database character set format. python-oracledb uses the type oracledb.DB_TYPE_CLOB.

  • NCLOB - National Character Large Object, used for string strings in the national character set format. python-oracledb uses the type oracledb.DB_TYPE_NCLOB.

  • BFILE - External Binary File, used for referencing a file stored on the host operating system outside of the database. python-oracledb uses the type oracledb.DB_TYPE_BFILE. See BFILEs for more information.

LOBs can be streamed to, and from, Oracle Database.

LOBs up to 1 GB in length can be also be handled directly as strings or bytes in python-oracledb. This makes LOBs easy to work with, and has significant performance benefits over streaming. However, it requires the entire LOB data to be present in Python memory, which may not be possible.

See GitHub for LOB examples.

11.1. Simple Insertion of LOBs

Consider a table with CLOB and BLOB columns:

CREATE TABLE lob_tbl (
    id NUMBER,
    c CLOB,
    b BLOB
);

With python-oracledb, LOB data can be inserted in the table by binding strings or bytes as needed:

with open('example.txt', 'r') as f:
    text_data = f.read()

with open('image.png', 'rb') as f:
    img_data = f.read()

cursor.execute("""
        insert into lob_tbl (id, c, b)
        values (:lobid, :clobdata, :blobdata)""",
        lobid=10, clobdata=text_data, blobdata=img_data)

Note that with this approach, LOB data is limited to 1 GB in size.

11.2. Fetching LOBs as Strings and Bytes

CLOBs and BLOBs smaller than 1 GB can queried from the database directly as strings and bytes. This can be much faster than streaming a LOB Object. Support is enabled by setting the Defaults Object.

import oracledb

# returns strings or bytes instead of a locator
oracledb.defaults.fetch_lobs = False

. . .

id_val = 1
text_data = "The quick brown fox jumps over the lazy dog"
binary_data = b"Some binary data"
cursor.execute("insert into lob_tbl (id, c, b) values (:1, :2, :3)",
               [id_val, text_data, binary_data])

cursor.execute("select c, b from lob_tbl where id = :1", [id_val])
clob_data, blob_data = cursor.fetchone()
print("CLOB length:", len(clob_data))
print("CLOB data:", clob_data)
print("BLOB length:", len(blob_data))
print("BLOB data:", blob_data)

This displays:

CLOB length: 43
CLOB data: The quick brown fox jumps over the lazy dog
BLOB length: 16
BLOB data: b'Some binary data'

An older alternative to using oracledb.defaults.fetch_lobs is to use a type handler:

def output_type_handler(cursor, metadata):
    if metadata.type_code is oracledb.DB_TYPE_CLOB:
        return cursor.var(oracledb.DB_TYPE_LONG, arraysize=cursor.arraysize)
    if metadata.type_code is oracledb.DB_TYPE_BLOB:
        return cursor.var(oracledb.DB_TYPE_LONG_RAW, arraysize=cursor.arraysize)
    if metadata.type_code is oracledb.DB_TYPE_NCLOB:
        return cursor.var(oracledb.DB_TYPE_LONG_NVARCHAR, arraysize=cursor.arraysize)

connection.outputtypehandler = output_type_handler

11.3. Streaming LOBs (Read)

Without setting oracledb.defaults.fetch_lobs to False, or without using an output type handler, the CLOB and BLOB values are fetched as LOB objects. The size of the LOB object can be obtained by calling LOB.size() and the data can be read by calling LOB.read():

id_val = 1
text_data = "The quick brown fox jumps over the lazy dog"
binary_data = b"Some binary data"
cursor.execute("insert into lob_tbl (id, c, b) values (:1, :2, :3)",
               [id_val, text_data, binary_data])

cursor.execute("select b, c from lob_tbl where id = :1", [id_val])
b, c = cursor.fetchone()
print("CLOB length:", c.size())
print("CLOB data:", c.read())
print("BLOB length:", b.size())
print("BLOB data:", b.read())

This approach produces the same results as the previous example but it will perform more slowly because it requires more round-trips to Oracle Database and has higher overhead. It is needed, however, if the LOB data cannot be fetched as one block of data from the server.

To stream the BLOB column, the LOB.read() method can be called repeatedly until all of the data has been read, as shown below:

cursor.execute("select b from lob_tbl where id = :1", [10])
blob, = cursor.fetchone()
offset = 1
num_bytes_in_chunk = 65536
with open("image.png", "wb") as f:
    while True:
        data = blob.read(offset, num_bytes_in_chunk)
        if data:
            f.write(data)
        if len(data) < num_bytes_in_chunk:
            break
        offset += len(data)

11.4. Streaming LOBs (Write)

If a row containing a LOB is being inserted or updated, and the quantity of data that is to be inserted or updated cannot fit in a single block of data, the data can be streamed using the method LOB.write() instead as shown in the following code:

id_val = 9
lob_var = cursor.var(oracledb.DB_TYPE_BLOB)
cursor.execute("""
        insert into lob_tbl (id, b)
        values (:1, empty_blob())
        returning b into :2""", [id_val, lob_var])
blob, = lobVar.getvalue()
offset = 1
num_bytes_in_chunk = 65536
with open("image.png", "rb") as f:
    while True:
        data = f.read(num_bytes_in_chunk)
        if data:
            blob.write(data, offset)
        if len(data) < num_bytes_in_chunk:
            break
        offset += len(data)
connection.commit()

11.5. Temporary LOBs

All of the examples shown thus far have made use of permanent LOBs. These are LOBs that are stored in the database. Oracle also supports temporary LOBs that are not stored in the database but can be used to pass large quantities of data. These LOBs use space in the temporary tablespace until all variables referencing them go out of scope or the connection in which they are created is explicitly closed.

When calling PL/SQL procedures with data that exceeds 32,767 bytes in length, python-oracledb automatically creates a temporary LOB internally and passes that value through to the procedure. If the data that is to be passed to the procedure exceeds that which can fit in a single block of data, however, you can use the method Connection.createlob() to create a temporary LOB. This LOB can then be read and written just like in the examples shown above for persistent LOBs.