5.8. Inheritance

PostgreSQL implements table inheritance which can be a useful tool for database designers. (SQL:1999 and later define a type inheritance feature, which differs in many respects from the features described here.)

Let's start with an example: suppose we are trying to build a data model for cities. Each state has many cities, but only one capital. We want to be able to quickly retrieve the capital city for any particular state. This can be done by creating two tables, one for state capitals and one for cities that are not capitals. However, what happens when we want to ask for data about a city, regardless of whether it is a capital or not? The inheritance feature can help to resolve this problem. We define the capitals table so that it inherits from cities:

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

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

In this case, the capitals table inherits all the columns of its parent table, cities. State capitals also have an extra column, 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 descendant tables. The latter behavior is the default. 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;

Given the sample data from the PostgreSQL tutorial (see Section 2.1), this 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 over 500ft:

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

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

Here the ONLY keyword indicates that the query should apply only to cities, and not any tables below cities in the inheritance hierarchy. Many of the commands that we have already discussed — SELECT, UPDATE and DELETE — support the ONLY keyword.

In some cases you may wish to know which table a particular row 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
----------+-----------+----------
   139793 | Las Vegas |     2174
   139793 | Mariposa  |     1953
   139798 | Madison   |      845

(If you try to reproduce this example, you will probably get different numeric OIDs.) By doing a join with pg_class you can see the actual table names:

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

Inheritance does not automatically propagate data from INSERT or COPY commands to other tables in the inheritance hierarchy. In our example, the following INSERT statement will fail:

INSERT INTO cities (name, population, altitude, state)
VALUES ('New York', NULL, NULL, 'NY');

We might hope that the data would somehow be routed to the capitals table, but this does not happen: INSERT always inserts into exactly the table specified. In some cases it is possible to redirect the insertion using a rule (see Chapter 34). However that does not help for the above case because the cities table does not contain the column state, and so the command will be rejected before the rule can be applied.

Check constraints can be defined on tables within an inheritance hierarchy. All check constraints on a parent table are automatically inherited by all of its children. Other types of constraints are not inherited, however.

A table can inherit from more than one parent table, in which case it has the union of the columns defined by the parent tables. Any columns declared in the child table's definition are added to these. If the same column name appears in multiple parent tables, or in both a parent table and the child's definition, then these columns are "merged" so that there is only one such column in the child table. To be merged, columns must have the same data types, else an error is raised. The merged column will have copies of all the check constraints coming from any one of the column definitions it came from.

Table inheritance can be defined using the CREATE TABLE statement using the INHERITS keyword. However the related statement CREATE TABLE AS does not allow inheritance to be specified.

Alternatively a table which is already defined in a compatible way can have a new parent added with ALTER TABLE using the INHERIT subform. To do this the new child table must already include columns with the same name and type as the columns of the parent. It must also include check constraints with the same name and check expression as those of the parent. Similarly an inheritance link can be removed from a child using the ALTER TABLE using the NO INHERIT subform.

One convenient way to create a compatible table to be a new child is using the LIKE option of CREATE TABLE. This creates a table with the same columns with the same type (however note the caveat below regarding constraints). Alternatively a compatible table can be created by first creating a new child using CREATE TABLE then removing the inheritance link with ALTER TABLE.

A parent table cannot be dropped while any of its children remain. If you wish to remove a table and all of its descendants, one easy way is to drop the parent table with the CASCADE option. Neither can columns of child tables be dropped or altered if they are inherited from any parent tables.

ALTER TABLE will propagate any changes in column data definitions and check constraints down the inheritance hierarchy. ALTER TABLE follows the same rules for duplicate column merging and rejection that apply during CREATE TABLE.