Re: Proposal: Add a UNIQUE NOT ENFORCED constraint

In many cases, the idiomatic/generally best way to write a query
requires a uniqueness check (a SELECT WHERE ... = ANY()/IN or really
any semi-join when optimized to an inner join, UNION, etc), meaning a
Unique/HashAggregate node will be added, increasing overhead unless
there is an explicit unique constraint. An unenforced unique
constraint would allow developers to use their knowledge of the
data/previous validation procedures to eliminate the extra node. This
would also help with documentation/observability tools by providing
more context on the data without adding overhead.