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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>5.9. Inheritance</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="ddl-schemas.html" title="5.8. Schemas" /><link rel="next" href="ddl-partitioning.html" title="5.10. Table Partitioning" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.9. Inheritance</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl-schemas.html" title="5.8. Schemas">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><th width="60%" align="center">Chapter 5. Data Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ddl-partitioning.html" title="5.10. Table Partitioning">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="DDL-INHERIT"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.9. Inheritance</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ddl-inherit.html#DDL-INHERIT-CAVEATS">5.9.1. Caveats</a></span></dt></dl></div><a id="id-1.5.4.11.2" class="indexterm"></a><a id="id-1.5.4.11.3" class="indexterm"></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
500 feet:
</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 class="xref" href="tutorial-sql-intro.html" title="2.1. Introduction">Section 2.1</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 500 feet:
</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 —
<code class="command">SELECT</code>, <code class="command">UPDATE</code> and
<code class="command">DELETE</code> — support the
<code class="literal">ONLY</code> keyword.
</p><p>
You can also write the table name with a trailing <code class="literal">*</code>
to explicitly specify that descendant tables are included:
</p><pre class="programlisting">
SELECT name, altitude
FROM cities*
WHERE altitude > 500;
</pre><p>
Writing <code class="literal">*</code> is not necessary, since this behavior is always
the default. However, this syntax is still supported for
compatibility with older releases where the default could be changed.
</p><p>
In some cases you might 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>
Another way to get the same effect is to use the <code class="type">regclass</code>
alias type, which will print the table OID symbolically:
</p><pre class="programlisting">
SELECT c.tableoid::regclass, c.name, c.altitude
FROM cities c
WHERE c.altitude > 500;
</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 ('Albany', 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 class="xref" href="rules.html" title="Chapter 41. The Rule System">Chapter 41</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>
All check constraints and not-null constraints on a parent table are
automatically inherited by its children, unless explicitly specified
otherwise with <code class="literal">NO INHERIT</code> clauses. Other types of constraints
(unique, primary key, and foreign key constraints) are not inherited.
</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">“<span class="quote">merged</span>”</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.
Inheritable check constraints and not-null constraints are merged in a
similar fashion. Thus, for example, a merged column will be marked
not-null if any one of the column definitions it came from is marked
not-null. Check constraints are merged if they have the same name,
and the merge will fail if their conditions are different.
</p><p>
Table inheritance is typically established when the child table is
created, using the <code class="literal">INHERITS</code> clause of the
<a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a>
statement.
Alternatively, a table which is already defined in a compatible way can
have a new parent relationship added, using the <code class="literal">INHERIT</code>
variant of <a class="xref" href="sql-altertable.html" title="ALTER TABLE"><span class="refentrytitle">ALTER TABLE</span></a>.
To do this the new child table must already include columns with
the same names and types as the columns of the parent. It must also include
check constraints with the same names and check expressions as those of the
parent. Similarly an inheritance link can be removed from a child using the
<code class="literal">NO INHERIT</code> variant of <code class="command">ALTER TABLE</code>.
Dynamically adding and removing inheritance links like this can be useful
when the inheritance relationship is being used for table
partitioning (see <a class="xref" href="ddl-partitioning.html" title="5.10. Table Partitioning">Section 5.10</a>).
</p><p>
One convenient way to create a compatible table that will later be made
a new child is to use the <code class="literal">LIKE</code> clause in <code class="command">CREATE
TABLE</code>. This creates a new table with the same columns as
the source table. If there are any <code class="literal">CHECK</code>
constraints defined on the source table, the <code class="literal">INCLUDING
CONSTRAINTS</code> option to <code class="literal">LIKE</code> should be
specified, as the new child must have constraints matching the parent
to be considered compatible.
</p><p>
A parent table cannot be dropped while any of its children remain. Neither
can columns or check constraints of child tables be dropped or altered
if they are inherited
from any parent tables. 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 (see <a class="xref" href="ddl-depend.html" title="5.13. Dependency Tracking">Section 5.13</a>).
</p><p>
<a class="xref" href="sql-altertable.html" title="ALTER TABLE"><span class="refentrytitle">ALTER TABLE</span></a> will
propagate any changes in column data definitions and check
constraints down the inheritance hierarchy. Again, dropping
columns that are depended on by other 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><p>
Inherited queries perform access permission checks on the parent table
only. Thus, for example, granting <code class="literal">UPDATE</code> permission on
the <code class="structname">cities</code> table implies permission to update rows in
the <code class="structname">capitals</code> table as well, when they are
accessed through <code class="structname">cities</code>. This preserves the appearance
that the data is (also) in the parent table. But
the <code class="structname">capitals</code> table could not be updated directly
without an additional grant. In a similar way, the parent table's row
security policies (see <a class="xref" href="ddl-rowsecurity.html" title="5.7. Row Security Policies">Section 5.7</a>) are applied to
rows coming from child tables during an inherited query. A child table's
policies, if any, are applied only when it is the table explicitly named
in the query; and in that case, any policies attached to its parent(s) are
ignored.
</p><p>
Foreign tables (see <a class="xref" href="ddl-foreign-data.html" title="5.11. Foreign Data">Section 5.11</a>) can also
be part of inheritance hierarchies, either as parent or child
tables, just as regular tables can be. If a foreign table is part
of an inheritance hierarchy then any operations not supported by
the foreign table are not supported on the whole hierarchy either.
</p><div class="sect2" id="DDL-INHERIT-CAVEATS"><div class="titlepage"><div><div><h3 class="title">5.9.1. Caveats</h3></div></div></div><p>
Note that not all SQL commands are able to work on
inheritance hierarchies. Commands that are used for data querying,
data modification, or schema modification
(e.g., <code class="literal">SELECT</code>, <code class="literal">UPDATE</code>, <code class="literal">DELETE</code>,
most variants of <code class="literal">ALTER TABLE</code>, but
not <code class="literal">INSERT</code> or <code class="literal">ALTER TABLE ...
RENAME</code>) typically default to including child tables and
support the <code class="literal">ONLY</code> notation to exclude them.
Commands that do database maintenance and tuning
(e.g., <code class="literal">REINDEX</code>, <code class="literal">VACUUM</code>)
typically only work on individual, physical tables and do not
support recursing over inheritance hierarchies. The respective
behavior of each individual command is documented in its reference
page (<a class="xref" href="sql-commands.html" title="SQL Commands">SQL Commands</a>).
</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 class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><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 class="listitem"><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 class="listitem"><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>
Some functionality not implemented for inheritance hierarchies is
implemented for declarative partitioning.
Considerable care is needed in deciding whether partitioning with legacy
inheritance is useful for your application.
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