Uniqueness

Two kinds: one column whose values never repeat, and a combination of columns whose pairs never repeat.

A unique column

Mark it Unique in the application, or list it alone in a constraint:

Schema:
  UniqueConstraints:
    - - 7e57d004-2b97-0e7a-b45f-5387367791cd

Distinct values are reproducible like everything else, and asking for nearly all of a column's possible values is no slower than asking for a few - there is no point where it starts retrying and grinds.

Checked before the run, not during

Every column type reports how many distinct values it can produce. Ask for more unique rows than that and you are told when you open or validate the schema:

Requested RowCount 1000 exceeds maximum unique combinations (12) for columns: [month]

This matters most for small domains - a pick list of three values, Weekday, Boolean - where the request is impossible rather than merely slow.

Combinations

List several ids in one constraint and the combination is unique, while each column repeats freely:

Schema:
  UniqueConstraints:
    - - 7e57d004-2b97-0e7a-b45f-5387367791cd    # region
      - 7e57d004-2b97-0e7a-b45f-5387367791ce    # month

12 months across 4 regions gives 48 distinct pairs, and asking for a 49th row fails validation.

What cannot be unique

Some types decline uniqueness: their domain is large but they cannot enumerate it, so producing distinct values on demand is not possible. The application disables the option for them rather than quietly producing duplicates.

A combination needs every column to have countable values

A combination is built by drawing from each column's set of values, so every column in it must have a countable set. Guid and Double do not, and neither does a Pick List drawing several values per row. Naming one in a combination is reported before the run starts:

The unique combination [id, month] cannot be enforced, because 'id' produces an
unbounded set of values and a combination is built by drawing from each column's
values. Remove it from the constraint, or mark it unique on its own.

On its own such a column is fine - a Guid never repeats, and marking it unique works exactly as you would expect. It is only the combination that cannot be built.

Optional columns

A column that is IsOptional leaves some cells blank, and blanks do not count as values for uniqueness. Combining a unique constraint with a low FillProbability is usually a mistake.