The Best Way to Handle LOBs in Oracle Distributed Databases

Handling Large Objects (LOBs) in Oracle distributed databases can be challenging. One of the common issues is the inability to directly access LOB columns in target tables when using database links (dblinks). This blog post will explore the problem in detail and provide effective strategies for managing LOBs efficiently.

Understanding the Problem

When you create a database link in Oracle, it allows you to access tables and data from a remote database. For example, you might create a dblink with a command like this:

create database link TEST_LINK 
  connect to TARGETUSER IDENTIFIED BY password using 'DATABASESID';

This dblink enables queries like the following to be executed:

select column_a, column_b 
from data_user.sample_table@TEST_LINK

However, if the column being accessed is a LOB, you could encounter the error:

ORA-22992: cannot use LOB locators selected from remote tables

This restriction is documented, and the suggested workaround involves fetching the LOB values into a local table, which often results in messy code and additional overhead.

Proposed Solution

1. Create a Temporary Table Approach

While the documentation suggests creating a temporary table to hold your LOB data, this method can become cumbersome. It often leads to additional complexity in managing these temporary tables. Here’s a basic version of how you might create a temporary table:

CREATE TABLE tmp_hello 
AS SELECT column_a 
FROM data_user.sample_table@TEST_LINK

However, this method can leave you with leftover temporary tables if an error occurs midway through your session.

2. Automate Temporary Table Management

To mitigate the messiness of handling temporary tables, consider encapsulating the table creation within a stored procedure. This encapsulation can simplify how clients interact with temporary tables. For instance, you might use the EXECUTE IMMEDIATE command within your procedure to create the table dynamically.

Example Stored Procedure

CREATE OR REPLACE PROCEDURE fetch_lob_data AS
BEGIN
    EXECUTE IMMEDIATE 'CREATE TABLE tmp_hello AS SELECT column_a FROM data_user.sample_table@TEST_LINK';
    -- Add your additional processing logic here
END;

3. Scheduled Clean-Up Jobs

One critical aspect of using temporary tables is ensuring that no “orphaned” tables are left behind if a session crashes. You can set up an Oracle job to periodically check for and remove any leftover tables:

BEGIN
    DBMS_SCHEDULER.create_job (
        job_name        => 'cleanup_temp_tables',
        job_type        => 'PLSQL_BLOCK',
        job_action      => 'BEGIN /* Add logic to drop left-over temporary tables */ END;',
        start_date      => SYSTIMESTAMP,
        repeat_interval  => 'FREQ=DAILY; BYHOUR=0; BYMINUTE=0; BYSECOND=0',
        enabled         => TRUE
    );
END;

This scheduled job can help you maintain a clean working environment while you tackle LOBs in your database.

Conclusion

Handling LOBs in Oracle distributed databases presents its own unique set of challenges, especially when using database links. While the documentation suggests a rather cumbersome workaround involving temporary tables, this can be streamlined with stored procedures and scheduled jobs for table clean-up. By implementing these strategies, you can effectively manage LOBs and maintain a more organized workspace.

By taking the time to properly set up your procedures and clean-up operations, you can simplify your process and reduce the risk of encountering issues associated with LOB handling.

If you have further insights or alternative methods, feel free to share them in the comments below!