What are the best practices for triggers to maintain a revision number on records? I have an audit trail implemented on an application database that captures old value, new value, modification time, and user inserting it into another table. I'm now wanting to add a revision number to each table to make idempondency checks more efficient.

As it stands, the trigger that captures all the data is AFTER UPDATE. My understanding is that  if I want to update the row I'm operating on, I will need to use a BEFORE trigger. ie:

CREATE TRIGGER `Table_A_Audit_Trigger` BEFORE UPDATE ON `Table_A`;
FOR EACH ROW BEGIN;
SET NEW.revision = OLD.revision + 1;


Would it be better to have two separate triggers for this, or to move the insert into the BEFORE trigger as well?

EDIT: The code for the AFTER UPDATE trigger (as well as the table DDL itself) is created by the application layer, then run on the database (generally when the model is created or updated in the software). It would not be difficult to add creation of this trigger at the same time.

As I understand it, the downside to combining into a BEFORE trigger would be inconsistency if the audit insert succeeds, but the actual update fails. Putting everything in an AFTER trigger would require the use of an additional UPDATE on the target table, which further compromises performance, and can cause cascading issues as well.
Along the lines of my response to your earlier question, surely this would be better dealt with in the stored procedure or insert/update query that is responsible for the change. That would be the logical place to increment a version column, not a trigger.

If a trigger is the only way you can be sure the revision count will be implemented, I'd suggest an INSTEAD OF trigger on any tables you need this. You can deal with the revision increment and auditing in one place then.