Chapter 8. Inheritance

Let's create two tables. The capitals table contains state capitals which are also cities. Naturally, the capitals table should inherit from cities.

CREATE TABLE cities (
    name            text,
    population      float,
    altitude        int     -- (in ft)
);

CREATE TABLE capitals (
    state           char(2)
) INHERITS (cities);

In this case, a row of capitals inherits all attributes (name, population, and altitude) from its parent, cities. The type of the attribute name is text, a native PostgreSQL type for variable length ASCII strings. The type of the attribute population is float, a native PostgreSQL type for double precision floating-point numbers. State capitals have an extra attribute, state, that shows their state. In PostgreSQL, a table can inherit from zero or more other tables, and a query can reference either all rows of a table or all rows of a table plus all of its descendants.

For example, the following query finds the names of all cities, including state capitals, that are located at an altitude over 500ft:

SELECT name, altitude
    FROM cities
    WHERE altitude > 500;
    

which returns:

+----------+----------+
|name      | altitude |
+----------+----------+
|Las Vegas | 2174     |
+----------+----------+
|Mariposa  | 1953     |
+----------+----------+
|Madison   | 845      |
+----------+----------+
   

On the other hand, the following query finds all the cities that are not state capitals and are situated at an altitude of 500ft or higher:

SELECT name, altitude
    FROM ONLY cities
    WHERE altitude > 500;

+----------+----------+
|name      | altitude |
+----------+----------+
|Las Vegas | 2174     |
+----------+----------+
|Mariposa  | 1953     |
+----------+----------+
   

Here the "ONLY" before cities indicates that the query should be run over only cities and not tables below cities in the inheritance hierarchy. Many of the commands that we have already discussed -- SELECT, UPDATE and DELETE -- support this "ONLY" notation.

In some cases you may wish to know which table a particular tuple originated from. There is a system column called TABLEOID in each table which can tell you the originating table:

    SELECT c.tableoid, c.name, c.altitude
    FROM cities c
    WHERE c.altitude > 500;
   

which returns:

+---------+----------+----------+
|tableoid |name      | altitude |
+---------+----------+----------+
|37292    |Las Vegas | 2174     |
+---------+----------+----------+
|37280    |Mariposa  | 1953     |
+---------+----------+----------+
|37280    |Madison   | 845      |
+---------+----------+----------+
   

If you do a join with pg_class you can see the actual table name:

    SELECT p.relname, c.name, c.altitude
    FROM cities c, pg_class p
    WHERE c.altitude > 500 and c.tableoid = p.oid;
   

which returns:

+---------+----------+----------+
|relname  |name      | altitude |
+---------+----------+----------+
|capitals |Las Vegas | 2174     |
+---------+----------+----------+
|cities   |Mariposa  | 1953     |
+---------+----------+----------+
|cities   |Madison   | 845      |
+---------+----------+----------+
   

A limitation of the inheritance feature is that indexes (including unique constraints) and foreign key constraints only apply to single tables, not to their inheritance children. Thus, in the above example, specifying that another table's column REFERENCES cities(name) would allow the other table to contain city names but not capital names. This deficiency will probably be fixed in some future release.