Create `before` and `after` triggers for queries

[renecannao]

mysql_query_rules can be extended with 2 more fields (names to be defined) that will execute commands before and after the executing of the real queries.
This could be useful, for example, to change the default of a session variable before and after executing a query.

[matthewvalimaki]

@renecannao first thank you for this awesome product.

What you proposed to yourself would be really great to have. A use case that comes to mind is application level encryption. Today this particular case could be satisfied by supporting MySQL commands within ProxySQL but I would think plugin/extension feature would be more beneficial. Doing simple exec from ProxySQL does not seem like a good idea. In terms of hashing or especially encryption doing them off the db would in heavy traffic environment shift load. It would probably be a good idea to implement circuit breaker for these so that bad implementations do not take down the whole proxy.

Slightly off-topic but here's an example how someone implemented something like this with Golang for Postgres https://github.com/go-pg/pg/wiki/Model-Hooks.