[WIP] Unnesting Arbitrary Subqueries

[jurplel]

Somewhere between a proof of concept and a draft—work still heavily in progress. Will already successfully parse and fully unnest a subset of correlated and uncorrelated subqueries (although I am uncertain about correctness).

TODO:

• [ ] Formal testing
• [ ] EXISTS clauses
• [ ] IN clauses
• [ ] ANY/ALL clauses
• [ ] Correctness issue with COUNT(*) (requires adding left outer join to plan)
• [x] Move some/all of this to rewriting stage to support multiple subqueries/ordering operations
• [x] “Sideways information passing” (subplans are duplicated now instead of making a DAG)
  • It seems that a DAG representation is only supported by looking for groups that appear the same. It looks to me that the cloned branches generated by this PR are indeed marked with the same group ID. I marked this bullet point as completed with this in mind.
• [ ] Support more pushdowns (e.g. limit, joins)
• [ ] Optimizations from the paper are all missing (Out of scope?)