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
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
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 —
DELETE — support the
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;
* 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;
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;
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
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 5.10).
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 5.13).
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
ALTER TABLE follows the same rules for duplicate column merging and rejection that apply during
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. Two exceptions to this rule are
LOCK TABLE, where permissions on the child tables are always checked, whether they are processed directly or recursively via those commands performed on the parent table.
In a similar way, the parent table's row security policies (see Section 5.7) 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 5.11) 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.,
DELETE, most variants of
ALTER TABLE, but not
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.,
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
PRIMARY KEY, this would not stop the
capitalstable 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
capitalswould 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
Similarly, if we were to specify that
REFERENCESsome other table, this constraint would not automatically propagate to
capitals. In this case you could work around it by manually adding the same
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.