DO $$
BEGIN
  IF NOT EXISTS (
    SELECT 1 FROM information_schema.table_constraints
    WHERE table_schema = '{{.SchemaName}}'
      AND table_name = '{{.TableName}}'
      AND constraint_name = '{{.ConstraintName}}'
  ) THEN
    ALTER TABLE {{quote_ident .SchemaName}}.{{quote_ident .TableName}} ADD CONSTRAINT {{quote_ident .ConstraintName}} UNIQUE ({{.Columns}});
  END IF;
END;
$$;