Drop updated_at column on audit_events

Problem to solve

Audit events should be append-only. The updated_at field is redundant and can be dropped to save storage spaces and reduce query payload.

Proposal

Drop updated_at column from audit_events table.

This change will be more likely to be spread across multiple releases (https://docs.gitlab.com/ee/development/what_requires_downtime.html#dropping-columns)

• 13.2: Ignore updated_at column and stop all writes
• 13.3: Remove updated_at column on audit_events
• 13.4: Clean up ignore column code in AuditEvent model

Further details

This will shave a shy (242,000,000 mil records * 8 bytes) in Gb = 2.2352 Gb

• https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9226#master-1022016106-18
• https://www.postgresql.org/docs/11/datatype-datetime.html

Availability & Testing

There is no known consumer of this field. Both UI and API only allow querying and presenting of created_at field.

Require collaboration with groupdatabase to understand if there is any risk.