Special case of #25123.

Feature request: Postgres supports ENUM types for defining columns that can contain only a given set of values. CockroachDB should, too.

https://www.postgresql.org/docs/10/static/datatype-enum.html

Special note:

Whenever a user-defined type is created (with CREATE TYPE), PostgreSQL automatically creates an associated array type, whose name consists of the element type's name prepended with an underscore, and truncated if necessary to keep it less than NAMEDATALEN bytes long. (If the name so generated collides with an existing type name, the process is repeated until a non-colliding name is found.) This implicitly-created array type is variable length and uses the built-in input and output functions array_in and array_out. The array type tracks any changes in its element type's owner or schema, and is dropped if the element type is.