Distributed EXPLAIN: Phase 3 - Complex queries

In phase 1-2, we only addressed simple queries without MapMerge Jobs. In this phase we nest EXPLAIN plans for nested jobs into the EXPLAIN plans of their parents. This requires more consideration of how to set up temporary tables and how to correctly implement the nesting.

Start by reviewing how distributed EXPLAIN handles the phase 1–2 simple-query cases and MapMerge Jobs. Investigate how temporary tables should be set up for nested jobs, and consider the work complete when nested EXPLAIN plans are correctly included in their parent plans.

Written by the indexing model from the issue text.