Optimizer plans for 10.4 - Long list

* Optimization for JOINs and ORDER BY (MDEV-8306 and co.)

* Condition pushdown into non-mergeable semi-join subqeries (we only have
  it for derived tables)

* Cost-based choice between materializing a derived table and merging it

* Support external references in Derived tables/CTEs (MDEV-13502)

* Reuse materialized CTEs (MDEV-11050) (MySQL 8.0 has this)

* Cost-based choice for splitting materialized derived tables
  (unconditional splitting is done for DBS, in 10.3)

* Filtering by Primary Key optimization

* Process single-table UPDATE/DELETE subquery as a semi-join (small task)

* A potential GSoC task: histograms based on a sample (like MySQL 8.0 does)
* Optimize for LIMIT when it is expressed through derived table, and ROW_NUMBER
  kind of window function (Oracle compatibility)

* A BIG Wish: Parallel execution [for some parts of the plan] (MDEV-6096)