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><H1
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><A
NAME="DDL-INHERIT"
>5.8. Inheritance</A
></H1
><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
<TT
CLASS="STRUCTNAME"
>capitals</TT
> table so that it inherits from
<TT
CLASS="STRUCTNAME"
>cities</TT
>:
</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 <TT
CLASS="STRUCTNAME"
>capitals</TT
> table <I
CLASS="FIRSTTERM"
>inherits</I
>
all the columns of its parent table, <TT
CLASS="STRUCTNAME"
>cities</TT
>. State
capitals also have an extra column, <TT
CLASS="STRUCTFIELD"
>state</TT
>, 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
HREF="tutorial-sql-intro.html"
>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 <TT
CLASS="LITERAL"
>ONLY</TT
> keyword indicates that the query
should apply only to <TT
CLASS="STRUCTNAME"
>cities</TT
>, and not any tables
below <TT
CLASS="STRUCTNAME"
>cities</TT
> in the inheritance hierarchy. Many
of the commands that we have already discussed —
<TT
CLASS="COMMAND"
>SELECT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
> and
<TT
CLASS="COMMAND"
>DELETE</TT
> — support the
<TT
CLASS="LITERAL"
>ONLY</TT
> keyword.
</P
><P
> You can also write the table name with a trailing <TT
CLASS="LITERAL"
>*</TT
>
to explicitly specify that descendant tables are included:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT name, altitude
FROM cities*
WHERE altitude > 500;</PRE
><P>
Writing <TT
CLASS="LITERAL"
>*</TT
> is not necessary, since this behavior is
the default (unless you have changed the setting of the
<A
HREF="runtime-config-compatible.html#GUC-SQL-INHERITANCE"
>sql_inheritance</A
> configuration option).
However writing <TT
CLASS="LITERAL"
>*</TT
> might be useful to emphasize that
additional tables will be searched.
</P
><P
> In some cases you might wish to know which table a particular row
originated from. There is a system column called
<TT
CLASS="STRUCTFIELD"
>tableoid</TT
> 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
<TT
CLASS="STRUCTNAME"
>pg_class</TT
> 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
<TT
CLASS="COMMAND"
>INSERT</TT
> or <TT
CLASS="COMMAND"
>COPY</TT
> commands to
other tables in the inheritance hierarchy. In our example, the
following <TT
CLASS="COMMAND"
>INSERT</TT
> 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
<TT
CLASS="STRUCTNAME"
>capitals</TT
> table, but this does not happen:
<TT
CLASS="COMMAND"
>INSERT</TT
> 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"
>Chapter 37</A
>). However that does not
help for the above case because the <TT
CLASS="STRUCTNAME"
>cities</TT
> table
does not contain the column <TT
CLASS="STRUCTFIELD"
>state</TT
>, 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. 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"
>"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, and will be marked not-null
if any of them are.
</P
><P
> Table inheritance is typically established when the child table is
created, using the <TT
CLASS="LITERAL"
>INHERITS</TT
> clause of the
<A
HREF="sql-createtable.html"
>CREATE TABLE</A
>
statement.
Alternatively, a table which is already defined in a compatible way can
have a new parent relationship added, using the <TT
CLASS="LITERAL"
>INHERIT</TT
>
variant of <A
HREF="sql-altertable.html"
>ALTER TABLE</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
<TT
CLASS="LITERAL"
>NO INHERIT</TT
> variant of <TT
CLASS="COMMAND"
>ALTER TABLE</TT
>.
Dynamically adding and removing inheritance links like this can be useful
when the inheritance relationship is being used for table
partitioning (see <A
HREF="ddl-partitioning.html"
>Section 5.9</A
>).
</P
><P
> One convenient way to create a compatible table that will later be made
a new child is to use the <TT
CLASS="LITERAL"
>LIKE</TT
> clause in <TT
CLASS="COMMAND"
>CREATE
TABLE</TT
>. This creates a new table with the same columns as
the source table. If there are any <TT
CLASS="LITERAL"
>CHECK</TT
>
constraints defined on the source table, the <TT
CLASS="LITERAL"
>INCLUDING
CONSTRAINTS</TT
> option to <TT
CLASS="LITERAL"
>LIKE</TT
> 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
<TT
CLASS="LITERAL"
>CASCADE</TT
> 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 that are depended on by other tables is only possible when using
the <TT
CLASS="LITERAL"
>CASCADE</TT
> option. <TT
CLASS="COMMAND"
>ALTER
TABLE</TT
> follows the same rules for duplicate column merging
and rejection that apply during <TT
CLASS="COMMAND"
>CREATE TABLE</TT
>.
</P
><P
> Note how table access permissions are handled. Querying a parent
table can automatically access data in child tables without further
access privilege checking. This preserves the appearance that the
data is (also) in the parent table. Accessing the child tables
directly is, however, not automatically allowed and would require
further privileges to be granted.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DDL-INHERIT-CAVEATS"
>5.8.1. Caveats</A
></H2
><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., <TT
CLASS="LITERAL"
>SELECT</TT
>, <TT
CLASS="LITERAL"
>UPDATE</TT
>, <TT
CLASS="LITERAL"
>DELETE</TT
>,
most variants of <TT
CLASS="LITERAL"
>ALTER TABLE</TT
>, but
not <TT
CLASS="LITERAL"
>INSERT</TT
> or <TT
CLASS="LITERAL"
>ALTER TABLE ...
RENAME</TT
>) typically default to including child tables and
support the <TT
CLASS="LITERAL"
>ONLY</TT
> notation to exclude them.
Commands that do database maintenance and tuning
(e.g., <TT
CLASS="LITERAL"
>REINDEX</TT
>, <TT
CLASS="LITERAL"
>VACUUM</TT
>)
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
HREF="sql-commands.html"
>Reference I, <I
>SQL Commands</I
></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
></P
></P><UL
><LI
><P
> If we declared <TT
CLASS="STRUCTNAME"
>cities</TT
>.<TT
CLASS="STRUCTFIELD"
>name</TT
> to be
<TT
CLASS="LITERAL"
>UNIQUE</TT
> or a <TT
CLASS="LITERAL"
>PRIMARY KEY</TT
>, this would not stop the
<TT
CLASS="STRUCTNAME"
>capitals</TT
> table from having rows with names duplicating
rows in <TT
CLASS="STRUCTNAME"
>cities</TT
>. And those duplicate rows would by
default show up in queries from <TT
CLASS="STRUCTNAME"
>cities</TT
>. In fact, by
default <TT
CLASS="STRUCTNAME"
>capitals</TT
> would have no unique constraint at all,
and so could contain multiple rows with the same name.
You could add a unique constraint to <TT
CLASS="STRUCTNAME"
>capitals</TT
>, but this
would not prevent duplication compared to <TT
CLASS="STRUCTNAME"
>cities</TT
>.
</P
></LI
><LI
><P
> Similarly, if we were to specify that
<TT
CLASS="STRUCTNAME"
>cities</TT
>.<TT
CLASS="STRUCTFIELD"
>name</TT
> <TT
CLASS="LITERAL"
>REFERENCES</TT
> some
other table, this constraint would not automatically propagate to
<TT
CLASS="STRUCTNAME"
>capitals</TT
>. In this case you could work around it by
manually adding the same <TT
CLASS="LITERAL"
>REFERENCES</TT
> constraint to
<TT
CLASS="STRUCTNAME"
>capitals</TT
>.
</P
></LI
><LI
><P
> Specifying that another table's column <TT
CLASS="LITERAL"
>REFERENCES
cities(name)</TT
> would allow the other table to contain city names, but
not capital names. There is no good workaround for this case.
</P
></LI
></UL
><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 application.
</P
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