Possibly detect adapter support for recursive queries

[danielkza]

Would it be feasible to attempt to detect support for recursive queries from the database adapter, an revert to a naive approach if it isn't available? This way it would be possible to use edge with SQLite and MySQL without needing any changes in client code.

I have two ideas on how this could be implemented:

1. Run a test query the first time a recursive query needs to be performed, and store whether it succeeded or not in a per-adapter module-wide variable. Then use that to decide what approach to use for subsequent queries.
2. Attempt to catch syntax errors on recursive query execution, fallback to the naive approach, and also cache the failure as above.

If this is desirable and not considered to hacky I can do it and send a PR.

[jackc]

It would be cool, but I think it may be harder than it initially appears.

For example, given the approach of trying a recursive query and recovering on error. What happens if the client code is already in a transaction? It could break the transaction and cause all future statements to fail until rollback. Possibly nested transactions or save points could solve this, but I do not know how widely they are supported. I think there will be a few other sharp edges as well.

So if you can get it to work, it would be a great feature, but getting it to work reliably may be tricky.