Allow users to create UNIQUE indexes on partition column for HASH and RANGE distributed tables.

[sumedhpathak]

Currently, we error out when users try to create UNIQUE indexes on distributed tables. However, we can allow users to create indexes on partition columns for HASH and RANGE distributed tables.

[ozgune]

Item #222 talks about enforcing these constraints at table creation time. We'd like for the user experience to be similar for ALTER TABLE changes.

Also, this issue could be related to #192.

[onderkalaci]

I think we should somehow unify the checks on create index and create table when considering the add this feature. Basically, there should be a single function which checks all these uniqeness constraints.

[sumedhpathak]

@mtuncer Could you look into this issue after your current tasks? @onderkalaci should be able to give you context on the change.

[mtuncer]

@sumedhpathak are we sure about enabling unique on range partitioned tables ? AFAIK we could not enforce disjointness on the range tables since one can create shards with overlapping ranges.

[onderkalaci]

@mtuncer But don't we have that assumption for all functionalities of range tables? Don't we expect the users to ensure that shards are non-overlapping for range partitioned tables?