Re: doc: Bring mention of unique index forced transaction wait behavior outside of the internal section

Sorry for the delayed response on this but I'm not a fan.  A comment of some form in transaction isolation seems to make sense (even if not my original thought...that patch got messed up a bit anyhow), and while having something in INSERT makes sense this doesn't seem precise enough.

Comments about locking and modifying rows doesn't make sense (the issue isn't relegated to ON CONFLICT, simple inserts will wait if they happen to choose the same key to insert).

I would also phrase it as simply "Tables with a unique index will..." and not even mention tables that lack a unique index - those don't really exist and inference of their behavior by contrast seems sufficient.

Sticking close to what you proposed then:

INSERT into tables with a unique index might block when concurrent sessions are inserting conflicting rows (i.e., have identical values for the unique index columns) or when there already exists a conflicting row which is in the process of being deleted.  Details are covered in <xref linkend="index-unique-checks"/>.

I can modify my original patch to be shorter and more on-point for inclusion in the MVCC chapter if there is interest in having a pointer from there to index-unique-checks as well.  I think such a note regarding concurrency on an index naturally fits into one of the main pages for learning about concurrency in PostgreSQL.

David J.