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GSOC 2020 - MDEV-383



I was looking into MDEV-383 <https://jira.mariadb.org/browse/MDEV-383>
- "*Evaluate
subquery predicates earlier or later depending on their SELECTIVITY*" as a
possible GSOC 2020 project and I have a few queries about the same.

   1. "There are a lot of subquery conditions out there that are
   inexpensive to evaluate and have good selectivity." - How is the cost of a
   subquery predicate calculated ?
   2. One of the assumptions in MDEV-83
   <https://jira.mariadb.org/browse/MDEV-83> was that "selectivity of a
   subquery predicate cannot be estimated during optimization." - what has
   changed since that we are looking into estimating selectivity ( or maybe my
   understanding of the task is incorrect so I would be grateful if you could
   just point me to any resources regarding this).
   3. Igor had responded on the jira - "for starters, you could introduce
   an option that would allow not to push correlated subquery predicates to
   joined tables." - did you mean a command line argument/option and I don't
   get how this would help with the main task.
   4. Also, can anyone please point me to the classes where predicate cost
   and join plans are evaluated ?

Apologies if the queries are a bit rudimentary.