Updates for Learned Indexes, NULLs are equal in DISTINCT but unequal in UNIQUE, fsyncgate & SERIAL is non-transactional.

[christoshrousis]

Added explanations in the commit messages, but most are self explanatory if you click through.

If it's not clear:

Learned Indexes appear to be a theoretical index, that hasn't been implemented in PostgreSQL, but is an interesting topic about the application of ML on datasets. The link provides many references.

The NULL one is pretty self explanatory, the link provides a table of how NULL is treated differently in different contexts.

fsyncgate points in a wikipedia page that outlines the whole history of fsyncgate.

SERIAL is technically non-transactional because when you rollback a transaction, SERIAL is still incremented outside of the transaction.