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<html>
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<title>5.8.Inheritance</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="ddl-inherit"></a>5.8.Inheritance</h2></div></div></div>
<a name="id577757"></a><a name="id577763"></a><p> <span class="productname">PostgreSQL</span> 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.)
</p>
<p> 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
<code class="structname">capitals</code> table so that it inherits from
<code class="structname">cities</code>:
</p>
<pre class="programlisting">CREATE TABLE cities (
name text,
population float,
altitude int -- in feet
);
CREATE TABLE capitals (
state char(2)
) INHERITS (cities);</pre>
<p>
In this case, the <code class="structname">capitals</code> table <em class="firstterm">inherits</em>
all the columns of its parent table, <code class="structname">cities</code>. State
capitals also have an extra column, <code class="structfield">state</code>, that shows
their state.
</p>
<p> In <span class="productname">PostgreSQL</span>, 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:
</p>
<pre class="programlisting">SELECT name, altitude
FROM cities
WHERE altitude > 500;</pre>
<p>
Given the sample data from the <span class="productname">PostgreSQL</span>
tutorial (see <a href="tutorial-sql.html#tutorial-sql-intro" title="2.1.Introduction">Section2.1, “Introduction”</a>), this returns:
</p>
<pre class="programlisting"> name | altitude
-----------+----------
Las Vegas | 2174
Mariposa | 1953
Madison | 845</pre>
<p>
</p>
<p> On the other hand, the following query finds all the cities that
are not state capitals and are situated at an altitude over 500ft:
</p>
<pre class="programlisting">SELECT name, altitude
FROM ONLY cities
WHERE altitude > 500;
name | altitude
-----------+----------
Las Vegas | 2174
Mariposa | 1953</pre>
<p>
</p>
<p> Here the <code class="literal">ONLY</code> keyword indicates that the query
should apply only to <code class="structname">cities</code>, and not any tables
below <code class="structname">cities</code> in the inheritance hierarchy. Many
of the commands that we have already discussed [mdash ]
<code class="command">SELECT</code>, <code class="command">UPDATE</code> and
<code class="command">DELETE</code> [mdash ] support the
<code class="literal">ONLY</code> keyword.
</p>
<p> In some cases you may wish to know which table a particular row
originated from. There is a system column called
<code class="structfield">tableoid</code> in each table which can tell you the
originating table:
</p>
<pre class="programlisting">SELECT c.tableoid, c.name, c.altitude
FROM cities c
WHERE c.altitude > 500;</pre>
<p>
which returns:
</p>
<pre class="programlisting"> tableoid | name | altitude
----------+-----------+----------
139793 | Las Vegas | 2174
139793 | Mariposa | 1953
139798 | Madison | 845</pre>
<p>
(If you try to reproduce this example, you will probably get
different numeric OIDs.) By doing a join with
<code class="structname">pg_class</code> you can see the actual table names:
</p>
<pre class="programlisting">SELECT p.relname, c.name, c.altitude
FROM cities c, pg_class p
WHERE c.altitude > 500 and c.tableoid = p.oid;</pre>
<p>
which returns:
</p>
<pre class="programlisting"> relname | name | altitude
----------+-----------+----------
cities | Las Vegas | 2174
cities | Mariposa | 1953
capitals | Madison | 845</pre>
<p>
</p>
<p> Inheritance does not automatically propagate data from
<code class="command">INSERT</code> or <code class="command">COPY</code> commands to
other tables in the inheritance hierarchy. In our example, the
following <code class="command">INSERT</code> statement will fail:
</p>
<pre class="programlisting">INSERT INTO cities (name, population, altitude, state)
VALUES ('New York', NULL, NULL, 'NY');</pre>
<p>
We might hope that the data would somehow be routed to the
<code class="structname">capitals</code> table, but this does not happen:
<code class="command">INSERT</code> always inserts into exactly the table
specified. In some cases it is possible to redirect the insertion
using a rule (see <a href="rules.html" title="Chapter34.The Rule System">Chapter34, <i>The Rule System</i></a>). However that does not
help for the above case because the <code class="structname">cities</code> table
does not contain the column <code class="structfield">state</code>, and so the
command will be rejected before the rule can be applied.
</p>
<p> 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.
</p>
<p> 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 “<span class="quote">merged</span>”
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.
</p>
<p> Table inheritance can currently only be defined using the <a href="sql-createtable.html">CREATE TABLE</a>
statement. The related statement <code class="command">CREATE TABLE AS</code> does
not allow inheritance to be specified. There
is no way to add an inheritance link to make an existing table into
a child table. Similarly, there is no way to remove an inheritance
link from a child table once it has been defined, other than by dropping
the table completely. 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 <code class="literal">CASCADE</code> option.
</p>
<p> <a href="sql-altertable.html">ALTER TABLE</a> will
propagate any changes in column data definitions and check
constraints down the inheritance hierarchy. Again, dropping
columns or constraints on parent tables is only possible when using
the <code class="literal">CASCADE</code> option. <code class="command">ALTER
TABLE</code> follows the same rules for duplicate column merging
and rejection that apply during <code class="command">CREATE TABLE</code>.
</p>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="ddl-inherit-caveats"></a>5.8.1.Caveats</h3></div></div></div>
<p> Table access permissions are not automatically inherited. Therefore,
a user attempting to access a parent table must either have permissions
to do the operation on all its child tables as well, or must use the
<code class="literal">ONLY</code> notation. When adding a new child table to
an existing inheritance hierarchy, be careful to grant all the needed
permissions on it.
</p>
<p> 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:
</p>
<div class="itemizedlist"><ul type="disc">
<li><p> If we declared <code class="structname">cities</code>.<code class="structfield">name</code> to be
<code class="literal">UNIQUE</code> or a <code class="literal">PRIMARY KEY</code>, this would not stop the
<code class="structname">capitals</code> table from having rows with names duplicating
rows in <code class="structname">cities</code>. And those duplicate rows would by
default show up in queries from <code class="structname">cities</code>. In fact, by
default <code class="structname">capitals</code> would have no unique constraint at all,
and so could contain multiple rows with the same name.
You could add a unique constraint to <code class="structname">capitals</code>, but this
would not prevent duplication compared to <code class="structname">cities</code>.
</p></li>
<li><p> Similarly, if we were to specify that
<code class="structname">cities</code>.<code class="structfield">name</code> <code class="literal">REFERENCES</code> some
other table, this constraint would not automatically propagate to
<code class="structname">capitals</code>. In this case you could work around it by
manually adding the same <code class="literal">REFERENCES</code> constraint to
<code class="structname">capitals</code>.
</p></li>
<li><p> Specifying that another table's column <code class="literal">REFERENCES
cities(name)</code> would allow the other table to contain city names, but
not capital names. There is no good workaround for this case.
</p></li>
</ul></div>
<p>
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.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Deprecated</h3>
<p> In previous versions of <span class="productname">PostgreSQL</span>, 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
standard. Under the old syntax, to include the child tables you append
<code class="literal">*</code> to the table name. For example:
</p>
<pre class="programlisting">SELECT * from cities*;</pre>
<p>
You can still explicitly specify scanning child tables by
appending <code class="literal">*</code>, as well as explicitly specify not
scanning child tables by writing <code class="literal">ONLY</code>. 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,
disable the <a href="runtime-config-compatible.html#guc-sql-inheritance">sql_inheritance</a> configuration
option.
</p>
</div>
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