给定一个PostgreSQL表requests，该表具有一个名为的列status和一个约束，如下所示：

ALTER TABLE requests ADD CONSTRAINT allowed_status_types
  CHECK (status IN (
    'pending', -- request has not been attempted
    'success', -- request succeeded
    'failure'  -- request failed
  ));

在这种情况下，psql我可以提取有关此约束的信息：

example-database=# \d requests
                                          Table "public.example-database"
        Column        |            Type             |                             Modifiers
----------------------+-----------------------------+-------------------------------------------------------------------
 id                   | integer                     | not null default nextval('requests_id_seq'::regclass)
 status               | character varying           | not null default 'pending'::character varying
 created_at           | timestamp without time zone | not null
 updated_at           | timestamp without time zone | not null

Indexes:
    "requests_pkey" PRIMARY KEY, btree (id)
Check constraints:
    "allowed_status_types" CHECK (status::text = ANY (ARRAY['pending'::character varying, 'success'::character varying, 'failure'::character varying]::text[]))

但是是否有可能编写一个专门返回allowed_status_types未决，成功，失败的查询？

能够在我的应用程序中记住此查询的结果，而不需要维护重复的副本，将是很棒的。