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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>5.10. 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 V1.79.1" /><link rel="prev" href="ddl-schemas.html" title="5.9. Schemas" /><link rel="next" href="ddl-partitioning.html" title="5.11. 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">5.10. Inheritance</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl-schemas.html" title="5.9. 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 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ddl-partitioning.html" title="5.11. 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">5.10. Inheritance</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ddl-inherit.html#DDL-INHERIT-CAVEATS">5.10.1. Caveats</a></span></dt></dl></div><a id="id-1.5.4.12.2" class="indexterm"></a><a id="id-1.5.4.12.3" 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,
    elevation       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 elevation over
   500 feet:

</p><pre class="programlisting">
SELECT name, elevation
    FROM cities
    WHERE elevation &gt; 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    | elevation
-----------+-----------
 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 elevation over 500 feet:

</p><pre class="programlisting">
SELECT name, elevation
    FROM ONLY cities
    WHERE elevation &gt; 500;

   name    | elevation
-----------+-----------
 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, elevation
    FROM cities*
    WHERE elevation &gt; 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.elevation
FROM cities c
WHERE c.elevation &gt; 500;
</pre><p>

   which returns:

</p><pre class="programlisting">
 tableoid |   name    | elevation
----------+-----------+-----------
   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.elevation
FROM cities c, pg_class p
WHERE c.elevation &gt; 500 AND c.tableoid = p.oid;
</pre><p>

   which returns:

</p><pre class="programlisting">
 relname  |   name    | elevation
----------+-----------+-----------
 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.elevation
FROM cities c
WHERE c.elevation &gt; 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, elevation, 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 40. The Rule System">Chapter 40</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="5.11. Table Partitioning">Section 5.11</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="5.14. Dependency Tracking">Section 5.14</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.  Two exceptions to this rule are
   <code class="command">TRUNCATE</code> and <code class="command">LOCK TABLE</code>,
   where permissions on the child tables are always checked,
   whether they are processed directly or recursively via those commands
   performed on the parent table.
  </p><p>
   In a similar way, the parent table's row
   security policies (see <a class="xref" href="ddl-rowsecurity.html" title="5.8. Row Security Policies">Section 5.8</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="5.12. Foreign Data">Section 5.12</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">5.10.1. 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.
  </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ddl-schemas.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ddl-partitioning.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5.9. Schemas </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 5.11. Table Partitioning</td></tr></table></div></body></html>