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5.5. Inheritance

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

Note: The inheritance hierarchy is actually a directed acyclic graph.

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 over 500ft:

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.

Deprecated: In previous versions of PostgreSQL, the default behavior was not to include child tables in queries. This was found to be error prone and is also in violation of the SQL:1999 standard. Under the old syntax, to get the sub-tables you append * to the table name. For example

SELECT * from cities*;

You can still explicitly specify scanning child tables by appending *, as well as explicitly specify not scanning child tables by writing "ONLY". But beginning in version 7.1, the default behavior for an undecorated table name is to scan its child tables too, whereas before the default was not to do so. To get the old default behavior, set the configuration option SQL_Inheritance to off, e.g.,

SET SQL_Inheritance TO OFF;

or add a line in your postgresql.conf file.

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

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 (plus any columns declared specifically for the child table).

A serious 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. This is true on both the referencing and referenced sides of a foreign key constraint. Thus, in the terms of the above example:

  • If we declared cities.name to be UNIQUE or a PRIMARY KEY, this would not stop the capitals table from having rows with names duplicating rows in cities. And those duplicate rows would by default show up in queries from cities. In fact, by default capitals would have no unique constraint at all, and so could contain multiple rows with the same name. You could add a unique constraint to capitals, but this would not prevent duplication compared to cities.

  • Similarly, if we were to specify that cities.name REFERENCES some other table, this constraint would not automatically propagate to capitals. In this case you could work around it by manually adding the same REFERENCES constraint to capitals.

  • Specifying that another table's column REFERENCES cities(name) would allow the other table to contain city names, but not capital names. There is no good workaround for this case.

These deficiencies will probably be fixed in some future release, but in the meantime considerable care is needed in deciding whether inheritance is useful for your problem.