Support WITH RECURSIVE queries

[Meyermagic]

https://www.postgresql.org/docs/current/static/queries-with.html https://www.sqlite.org/lang_with.html

Blocked on #33.

[jchillin]

Now that #33 has been fixed by #2514, what are the chances this issue gets resolved?

[weiznich]

@jchillin We do not have any concrete plans to implement this soon, but we are open for contributions to the required design work and implementation afterwards. Feel free to submit a concrete proposal outlining a potential API as new feature request.

[Sytten]

I would be happy to contribute but this is some advanced stuff for me, do you have hints of where I should start looking? Thanks!

[weiznich]

@Sytten We are happy to receive contributions here, but be warned that this is likely a feature that requires quite a bit of work to implement is completely.

So after putting that disclaimer first here is what needs to be done:

First we need to agree on a design how we would like to expose this feature via diesels dsl. That's something we haven't talked about or even looked into yet. So someone needs to sit down and write a proposal how a API could look like and how we could check specific parts of the invariants involved with such queries. Such a proposal should be submitted here. You don't need to bring in the whole proposed API as rust code there, but I would expect that to answer at least questions like: Should this API be exposed as trait, struct, function or macro? Which invariant exist? How can those invariants be checked at compile time? After we've discussed that we can talk about the actual implementation.