gitlab-ce/doc/development/creating_enums.md

Creating enums

When creating a new enum, it should use the database type SMALLINT. The SMALLINT type size is 2 bytes, which is sufficient for an enum. This would help to save space in the database.

To use this type, add limit: 2 to the migration that creates the column.

Example:

def change
  add_column :ci_job_artifacts, :file_format, :integer, limit: 2
end