[ remembering previous discussions more clearly... ]  Actually there
is a concrete problem here: unique constraints are supposed to be
represented in the information_schema views, and there is no
spec-compliant way to do that for a constraint on something other than
a column.  We'd have to guess at what the SQL committee would do about
that, and the odds of guessing exactly right don't seem encouraging.

Considering that a unique index is a unique constraint, then isn't allowing
expressional unique indexes contradictory to the above?

It seems that "CREATE UNIQUE INDEX" currently does not pass isconstraint as
true to DefineIndex, otherwise  index_create() would have cribbed with:

"constraints cannot have index expressions" error.

Since this patch is going to consider creating unique/primary indexes
assuming them to be constraints, IMHO we should be uniform about unique
constraints semantics.

That might mean that we only support expressionless, non-predicate indexes


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