PostgreSQL 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.)
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
capitals
table so that it inherits from
cities
:
CREATE TABLE cities ( name text, population float, elevation int -- in feet ); CREATE TABLE capitals ( state char(2) ) INHERITS (cities);
In this case, the capitals
table inherits
all the columns of its parent table, cities
. State
capitals also have an extra column, 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 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:
SELECT name, elevation FROM cities WHERE elevation > 500;
Given the sample data from the PostgreSQL tutorial (see Section 2.1), this returns:
name | elevation -----------+----------- 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 elevation over 500 feet:
SELECT name, elevation FROM ONLY cities WHERE elevation > 500; name | elevation -----------+----------- Las Vegas | 2174 Mariposa | 1953
Here the ONLY
keyword indicates that the query
should apply only to cities
, and not any tables
below cities
in the inheritance hierarchy. Many
of the commands that we have already discussed —
SELECT
, UPDATE
and
DELETE
— support the
ONLY
keyword.
You can also write the table name with a trailing *
to explicitly specify that descendant tables are included:
SELECT name, elevation FROM cities* WHERE elevation > 500;
Writing *
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.
In some cases you might 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.elevation FROM cities c WHERE c.elevation > 500;
which returns:
tableoid | name | elevation ----------+-----------+----------- 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.elevation FROM cities c, pg_class p WHERE c.elevation > 500 AND c.tableoid = p.oid;
which returns:
relname | name | elevation ----------+-----------+----------- cities | Las Vegas | 2174 cities | Mariposa | 1953 capitals | Madison | 845
Another way to get the same effect is to use the regclass
alias type, which will print the table OID symbolically:
SELECT c.tableoid::regclass, c.name, c.elevation FROM cities c WHERE c.elevation > 500;
Inheritance does not automatically propagate data from
INSERT
or COPY
commands to
other tables in the inheritance hierarchy. In our example, the
following INSERT
statement will fail:
INSERT INTO cities (name, population, elevation, state) VALUES ('Albany', NULL, NULL, 'NY');
We might hope that the data would somehow be routed to the
capitals
table, but this does not happen:
INSERT
always inserts into exactly the table
specified. In some cases it is possible to redirect the insertion
using a rule (see Chapter 41). However that does not
help for the above case because the cities
table
does not contain the column state
, and so the
command will be rejected before the rule can be applied.
All check constraints and not-null constraints on a parent table are
automatically inherited by its children, unless explicitly specified
otherwise with NO INHERIT
clauses. Other types of constraints
(unique, primary key, and foreign key constraints) are not inherited.
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 “merged” 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.
Table inheritance is typically established when the child table is
created, using the INHERITS
clause of the
CREATE TABLE
statement.
Alternatively, a table which is already defined in a compatible way can
have a new parent relationship added, using the INHERIT
variant of ALTER TABLE.
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
NO INHERIT
variant of ALTER TABLE
.
Dynamically adding and removing inheritance links like this can be useful
when the inheritance relationship is being used for table
partitioning (see Section 5.11).
One convenient way to create a compatible table that will later be made
a new child is to use the LIKE
clause in CREATE
TABLE
. This creates a new table with the same columns as
the source table. If there are any CHECK
constraints defined on the source table, the INCLUDING
CONSTRAINTS
option to LIKE
should be
specified, as the new child must have constraints matching the parent
to be considered compatible.
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
CASCADE
option (see Section 5.14).
ALTER TABLE 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 CASCADE
option. ALTER
TABLE
follows the same rules for duplicate column merging
and rejection that apply during CREATE TABLE
.
Inherited queries perform access permission checks on the parent table
only. Thus, for example, granting UPDATE
permission on
the cities
table implies permission to update rows in
the capitals
table as well, when they are
accessed through cities
. This preserves the appearance
that the data is (also) in the parent table. But
the capitals
table could not be updated directly
without an additional grant. In a similar way, the parent table's row
security policies (see Section 5.8) 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.
Foreign tables (see Section 5.12) 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.
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., SELECT
, UPDATE
, DELETE
,
most variants of ALTER TABLE
, but
not INSERT
or ALTER TABLE ...
RENAME
) typically default to including child tables and
support the ONLY
notation to exclude them.
Commands that do database maintenance and tuning
(e.g., REINDEX
, VACUUM
)
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 (SQL Commands).
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.
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.