FeatureRequest: get_pages/select_pages

[mattalbr]

I use sqlakeyset to do keyset-based pagination in my GraphQL server. It works great for top-level resolvers, but if I have nested pages, I end up encountering the N + 1 GraphQL problem. The solution to the N + 1 problem is basically a "DataLoader" that just implements a batch API. Functionally, as it relates to sqlakeyset, that means implementing a "get_pages".

I went ahead and implemented this for my company, and was hoping that sqlakeyset would be receptive to homing this functionality. I'd be happy to send a PR.

More specifically, I implemented a get_homogeneous_pages that assumes the queries all select the same columns (but can have different filters or order_bys), which allows us to do a UNION ALL and make a single round trip to the database. That would be an easy addition to sqlakeyset because there's no need to understand the caller's nor session's threading model. A get_heterogeneous_pages may be useful to someone else, but requires making assumptions about how to execute asynchronously that I think sqlakeyset probably shouldn't touch.

Thoughts? Would you be open to adding this if I sent a PR?

[acarapetis]

Feel free to open the PR, I'll have a look when I can :)