Caching query plan costs

This email thread from last year:

    https://www.postgresql.org/message-id/flat/CAMjNa7cC4X9YR-vAJS-jSYCajhRDvJQnN7m2sLH1wLh-_Z2bsw%40mail.gmail.com

got stuck on when to check for certain optimizations in the planner.  It
was discussed that knowing a plan was going to be expensive _before_
planning could help determine when to try certain expensive
optimizations.

What if we globally or locally cache the _cost_ of plans, so we can
consult that cache before planning and enable certain optimizations? It
also might be useful to cache some of the unexpected things found during
execution, e.g. inaccurate cost estimates.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +