SQL: Optimizations

[joshua-spacetime]

We will implement the following rule-based optimizations:

1. Filter pushdown
   In particular pushing filters below joins. In the logical plan, WHERE conditions follow joins and will need to be pushed below the join in order to perform index selection.

2. Join selection
   In the logical plan, joins are n-ary and are initially translated as cross joins. We need to optimize them as index joins if possible.

3. Table scan + filter -> index scan
   After pushing down filters, we need to generate applicable index scans.

4. Project pull up
   For both sql and subscriptions, the logical plan may have intermediate projections, but these can either be removed entirely, as is the case for subscriptions, or pulled up to the root of the query plan, as is the case for sql, to avoid materializing product values.