在Postgresql中，是否有一种方法可以将列的值限制为枚举？

Question

In postgresql , I can create a table documenting which type of vehicle people have.

CREATE TABLE IF NOT EXISTS person_vehicle_type
( id SERIAL NOT NULL PRIMARY KEY
, name TEXT NOT NULL
, vehicle_type TEXT
);

This table might have values such as

 id | name    | vehicle_type
----+---------+---------
  1 | Joe     | sedan
  2 | Sue     | truck
  3 | Larry   | motorcycle
  4 | Mary    | sedan
  5 | John    | truck
  6 | Crystal | motorcycle
  7 | Matt    | sedan

The values in the car_type column are restricted to the set { sedan , truck , motorcycle }.

Is there a way to formalize this restriction in postgresql ?

Answer 1

Personally I would use foreign key and lookup table.

Anyway you could use enums. I recommend to read article PostgreSQL Domain Integrity In Depth :

A few RDBMSes (PostgreSQL and MySQL) have a special enum type that ensures a variable or column must be one of a certain list of values. This is also enforcible with custom domains.

However the problem is technically best thought of as referential integrity rather than domain integrity, and usually best enforced with foreign keys and a reference table. Putting values in a regular reference table rather than storing them in the schema treats those values as first-class data. Modifying the set of possible values can then be performed with DML (data manipulation language) rather than DDL (data definition language) ....

However when the possible enumerated values are very unlikely to change, then using the enum type provides a few minor advantages.

1. Enums values have human-readable names but internally they are simple integers. They don't take much storage space. To compete with this efficiency using a reference table would require using an artificial integer key, rather than a natural primary key of the value description. Even then the enum does not require any foreign key validation or join query overhead.

2. Enums and domains are enforced everywhere, even in stored procedure arguments, whereas lookup table values are not. Reference table enumerations are enforced with foreign keys, which apply only to rows in a table.

3. The enum type defines an automatic (but customizable) order relation:

 CREATE TYPE log_level AS ENUM ('notice', 'warning', 'error', 'severe');
 CREATE TABLE log(i SERIAL, level log_level);
 INSERT INTO log(level) 
 VALUES ('notice'::log_level), ('error'::log_level), ('severe'::log_level);

 SELECT * FROM log WHERE level >= 'warning';

DBFiddle Demo

Drawback:

Unlike a restriction of values enforced by foreign key, there is no way to delete a value from an existing enum type. The only workarounds are messing with system tables or renaming the enum, recreating it with the desired values, then altering tables to use the replacement enum. Not pretty.