Inheritance

PostgreSQL 8.0.1 Documentation
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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.

    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. 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.

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