The batching query used to flush the outbox is doing a sort on `created_on` column to process the records in the order they were created. Without the index the query will take too long even with a small batch size.

Idea for how to do this:

• Create a sql script that will put 3.5m placeholder records in the database table
• Run SQL query to fetch, note the time

SELECT * FROM hbi_event_outbox ORDER BY created_on ASC LIMIT 1000

Note: we can play with adjusting the limit 1000 to get better performance

• Add index to the table and repeat the process, note the time.

Acceptance Criteria

• Changeset created to add the index to the table.
• Performance check to ensure that the index improves the query time.
• Performance check on the liquibase update, given that there are 3.5 million records currently in stage.