Inheritance is a concept from object-oriented databases. It opens up interesting new possibilities of database design.
    Let's create two tables:  A table cities
    and a table capitals.  Naturally, capitals
    are also cities, so you want some way to show the capitals
    implicitly when you list all cities.  If you're really clever you
    might invent some scheme like this:
CREATE TABLE capitals (
    name            text,
    population      real,
    altitude        int,    -- (in ft)
    state           char(2)
);
CREATE TABLE non_capitals (
    name            text,
    population      real,
    altitude        int     -- (in ft)
);
CREATE VIEW cities AS
    SELECT name, population, altitude FROM capitals
        UNION
    SELECT name, population, altitude FROM non_capitals;This works OK as far as querying goes, but it gets ugly when you need to update several rows, to name one thing.
A better solution is this:
CREATE TABLE cities (
    name            text,
    population      real,
    altitude        int     -- (in ft)
);
CREATE TABLE capitals (
    state           char(2)
) INHERITS (cities);
    In this case, a row of capitals
    inherits all columns (name,
    population, and altitude) from its
    parent, cities.  The
    type of the column name is
    text, a native PostgreSQL
    type for variable length character strings.  State capitals have
    an extra column, state, that shows their state.  In
    PostgreSQL, a table can inherit from
    zero or more other tables.
   
For example, the following query finds the names of all cities, including state capitals, that are located at an altitude over 500 ft.:
SELECT name, altitude
    FROM cities
    WHERE altitude > 500;which returns:
name | altitude -----------+---------- Las Vegas | 2174 Mariposa | 1953 Madison | 845 (3 rows)
On the other hand, the following query finds all the cities that are not state capitals and are situated at an altitude of 500 ft. or higher:
SELECT name, altitude
    FROM ONLY cities
    WHERE altitude > 500;
name | altitude -----------+---------- Las Vegas | 2174 Mariposa | 1953 (2 rows)
    Here the ONLY before cities
    indicates that the query should be run over only the
    cities table, 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.