Primary Key vs Unique Keys redundancy

[dmzolotarev]

The current spec and examples show redundancy between primary_key and unique_keys:

# From spec.md example (lines 126-128)
primary_key: [order_id]
unique_keys:
  - [order_id]      # Duplicates primary_key
  - [order_number]

Should it be duplicated in unique_keys, since primary key should always be a unique key as well?

We could either:

• explicitly document that primary_key is implicitly treated as a unique key and should NOT be duplicated.
• consider merging primary_key and unique_keys into a single keys structure with a primary: true flag?