=head1 NAME
Tangram - Mapping inheritance
=head1 DESCRIPTION
There are many ways of representing inheritance relationships in a relational
database. This document describes three popular ways and how Tangram supports
them.
=head1 STRATEGIES FOR MAPPING INHERITANCE
Inheritance is a concept that has no equivalent in the relational
world. However, it is possible to implement it by using a combination of
relational features like tables and foreign keys.
One of the paramount issues about mapping inheritance is how well the mapping
supports polymorphism. Any Object-Oriented persistence facility that deserves
its name needs to allow the retrieval of all the Fruits, and return a
heterogeneous collection of Apples, Oranges and Bananas. Also, it must
perform this operation in an efficient manner. In particular, polymorphic
retrieval should not cost one SELECT per retrieved object.
A secondary - yet important - issue is how well the mapping plays by the rules
of orthogonal orthodoxy.
Another issue we'll examine is how well the mapping supports 'complex' queries,
that is, queries that involve several objects
Three strategies are in common use, that go by the name Vertical, Horizontal
and Filtered mapping. They all have advantages and disadvantages.
The following sections describe the three strategies in details. They make use of
a simple object model to illustrate the mappings.
+---------------------+
| Person |
| {abstract} |
+---------<------- 1 +---------------------+
| | name: string |
| +---------------------+
| |
| ^
| |
| +------------------+---------------------+
| | |
| +---------------+ +-----------------+
V | NaturalPerson | | LegalPerson |
| +---------------+ +-----------------+
| | age: integer | | form: string |
| +---------------+ +-----------------+
|
|
|
| +---------------------+
+-------->-------- * | Vehicle |
| {abstract} |
+---------------------+
| make: string |
+---------------------+
|
^
|
+------------------+-------------------+
| |
+---------------+ +-----------------+
| Car | | Plane |
+---------------+ +-----------------+
| plate: string | | ident: string |
+---------------+ +-----------------+
=head1 Horizontal Mapping
=head1 description
Each I<concrete> class is mapped onto a single table. Each row in the
table describes the persistent state of one object.
The attributes are mapped onto columns, usually one column per
attribute but not necessarily. For example, collections may be stored elsewhere
(for example on a link table) and thus require no column on the class' table.
In effect, the database looks like this:
+---------------+
| NaturalPerson |
+------+--------+-------+------+
| id | name | age |
================================
| 17 | Bill Gates | 46 |
+------+----------------+------+
| 23 | Georges Bush | 50 |
+------+----------------+------+
+-------------+
| LegalPerson |
+------+------+---------+------+
| id | name | form |
================================
| 36 | Microsoft | Inc |
+------+----------------+------+
+------+
| Car |
+------+-------+----------------+--------+
| id | owner | make | plate |
==========================================
| 12 | 17 | Saab | BILL-1 |
+------+-------+----------------+--------+
| 50 | 36 | Miata | MS-001 |
+------+-------+----------------+--------+
| 51 | 36 | Miata | MS-002 |
+------+-------+----------------+--------+
+-------+
| Plane |
+------++-----+----------------+--------+
| id | owner| make | ident |
=========================================
| 29 | 23 | Boeing | AF-001 |
+------+------+----------------+--------+
=head2 advantages
Polymorphic retrieval costs one SELECT per concrete conforming class; retrieving
all the Persons costs two SELECTs. These SELECTs, however, don't use joins -
an expensive operation. In our example, retrieving all the Persons requires the
following two SELECTs:
SELECT id, name, age FROM NaturalPerson
SELECT id, name, form FROM LegalPerson
=head2 disadvantages
This mapping is reasonable with regard to relational orthodoxy, but not perfect:
the 'name' column is present on two different tables, with the same semantic.
The biggest drawback, however, happens when you try to perfrom complex queries.
Suppose oyu want to retrieve all the Persons (Natural- or Legal-) that own a
Vehicle of make 'Saab' (be it a Car or a Plane). Sticking with equijoins, the
cost of the operation is four SELECTs:
SELECT NaturalPerson.id, NaturalPerson.name, NaturalPerson.age
FROM NaturalPerson, Car
WHERE Car.owner = NaturalPerson.id
SELECT NaturalPerson.id, NaturalPerson.name, NaturalPerson.age
FROM NaturalPerson, Plane
WHERE Plane.owner = NaturalPerson.id
SELECT LegalPerson.id, LegalPerson.name, LegalPerson.form
FROM LegalPerson, Car
WHERE Car.owner = LegalPerson.id
SELECT LegalPerson.id, LegalPerson.name, LegalPerson.form
FROM LegalPerson, Plane
WHERE Plane.owner = LegalPerson.id
When the depth of the hierarchies increase, the combinatory explosion makes
complex queries prohibitive.
=head1 Vertical Mapping
=head2 description
Each class has its corresponding table, which contains only the class' direct
fields. In other words, the table doesn't store the inherited fields. Both
concrete and abstract classes get a table. The state of an object is
thus scattered over several tables.
For example:
+--------+
| Person |
+------+-+------+-------+
| id | name |
=========================
| 17 | Bill Gates |
+------+----------------+
| 23 | Georges Bush |
+------+----------------+
| 36 | Microsoft |
+------+----------------+
+---------------+ +-------------+
| NaturalPerson | | LegalPerson |
+------+--------+ +-------+-----++
| id | age | | id | form |
================= ================
| 17 | 46 | | 36 | Inc |
+------+--------+ +-------+------+
| 23 | 50 |
+------+--------+
+---------+
| Vehicle |
+------+--+----+----------------+
| id | owner | make |
=================================
| 12 | 17 | Saab |
+------+-------+----------------+
| 29 | 23 | AF-001 |
+------+-------+----------------+
| 50 | 36 | Miata |
+------+-------+----------------+
| 51 | 36 | Miata |
+------+-------+----------------+
+------+ +-------+
| Car | | Plane |
+------++--------+ +-------+--------+
| id | plate | | id | ident |
================== ==================
| 12 | BILL-1 | | 29 | AF-001 |
+-------+--------+ +-------+--------+
| 50 | MS-001 |
+-------+--------+
| 51 | MS-002 |
+-------+--------+
Polymorphic retrieval is achieved by issuing one SELECT per concrete
conforming class; retrieving In our example, retrieving all the
Persons requires the following two SELECTs:
SELECT Person.id, Person.name, NaturalPerson.age
FROM Person, NaturalPerson
WHERE Person.id = NaturalPerson.id
SELECT Person.id, Person.name, LegalPerson.form
FROM Person, LegalPerson
WHERE Person.id = LegalPerson.id
This mapping sometimes needs an extra column that carries a type
identifier. In our example, we take the very resonable assumption that
Person is an abstract class. Had we decided to allow 'pure' Persons,
we would have been faced with the following problem: the Person table
would contain rows that describe pure Persons, but also rows that
describe the Person part of Natural- and LegalPersons. We would need
to filter those incomplete objects out when retrieving the pure
Persons. Thus the Person table would look like this:
+--------+
| Person |
+-----+--+---+----------------+
| id | type | name |
===============================
| 13 | 1 | Pure Person |
+-----+------+----------------+
| 17 | 2 | Bill Gates |
+-----+------+----------------+
| 23 | 2 | Georges Bush |
+-----+------+----------------+
| 36 | 3 | Microsoft |
+-----+------+----------------+
In this case, we need an extra SELECT for retrieving pure Persons:
SELECT Person.id, Person.name
FROM Person
WHERE Person.type IN (1)
=head2 advantages
From the relational point of view, this mapping is excellent: the
resulting database is in third normal form.
This mapping also supports complex queries very well. Take the Saab
owners example again: we don't need to involve the Car nor Plane
tables in the query. As a result, two SELECTs suffice:
SELECT Person.id, Person.name, NaturalPerson.age
FROM Person, NaturalPerson, Vehicle
WHERE Person.id = NaturalPerson.id AND Vehicle.owner = Person.id
SELECT Person.id, Person.name, LegalPerson.form
FROM Person, LegalPerson, Vehicle
WHERE Person.id = LegalPerson.id AND Vehicle.owner = Person.id
=head2 disadvantages
The mapping potentially has the highest performance cost: it requires
multiple SELECTs like the horizontal mapping, but in addition, these
SELECTs use joins.
=head1 Filtered Mapping
=head2 description
Entire hierarchies are mapped onto a single table. Two rows may
describe objects of different types, maybe completely unrelated. The
set of columns is the uperset of all the columns needed by all the
attributes of any of the classes involved in the mapping.
A special 'type' column contains an value that uniquely identifies the
concrete class of the object described by the row.
All the columns related to attributes that don't occur in all the
classes must be declared as NULLABLE. Indeed, the table may contain
mostly NULL values.
In our example, the database may look either like this:
+---------+
| Persons |
+-----+---+--+----------------+------+------+
| id | type | name | age | form |
=============================================
| 17 | 1 | Bill Gates | 46 | NULL |
+-----+------+----------------+------+------+
| 23 | 1 | Georges Bush | 50 | NULL |
+-----+------+----------------+------+------+
| 36 | 2 | Microsoft | NULL | Inc |
+-----+------+----------------+------+------+
+---------+
| Persons |
+-----+---+--+----------------+------+------+
| id | type | name | age | form |
=============================================
| 17 | 1 | Bill Gates | 46 | NULL |
+-----+------+----------------+------+------+
| 23 | 1 | Georges Bush | 50 | NULL |
+-----+------+----------------+------+------+
| 36 | 2 | Microsoft | NULL | Inc |
+-----+------+----------------+------+------+
| 36 | 2 | Microsoft | NULL | Inc |
+-----+------+----------------+------+------+
+----------+
| Vehicles |
+-----+----+-+-------+----------------+--------+--------+
| id | type | owner | make | plate | ident |
=========================================================
| 12 | 3 | 17 | Saab | BILL-1 | NULL |
+-----+------+-------+----------------+--------+--------+
| 29 | 4 | 23 | Boeing | NULL | AF-001 |
+-----+------+-------+----------------+--------+--------+
| 50 | 3 | 36 | Miata | MS-001 | NULL |
+-----+------+-------+----------------+--------+--------+
| 51 | 3 | 36 | Miata | MS-002 | NULL |
+-----+------+-------+----------------+--------+--------+
Retrieving all the Persons requires only one SELECT:
SELECT id, name, age, form FROM Persons
When retrieving NaturalPersons we must take care to filter out the
rows that belog to LegalPersons:
SELECT id, name, age FROM Persons WHERE type = 1
We may even decide to place unrelated hierarchies on the same table:
+---------+
| Objects |
+-----+---+--+---------------+------+------+--------+--------+--------+
| id | type | name | age | form | make | plate | ident |
=======================================================================
| 17 | 1 | Bill Gates | 46 | NULL | NULL | NULL | NULL |
+-----+------+---------------+------+------+--------+--------+--------+
| 23 | 1 | Georges Bush | 50 | NULL | NULL | NULL | NULL |
+-----+------+---------------+------+------+--------+--------+--------+
| 36 | 2 | Microsoft | NULL | Inc | NULL | NULL | NULL |
+-----+------+---------------+------+------+--------+--------+--------+
| 12 | 3 | NULL | NULL | NULL | Saab | BILL-1 | NULL |
+-----+------+---------------+------+------+--------+--------+--------+
| 29 | 4 | NULL | NULL | NULL | Boeing | NULL | AF-001 |
+-----+------+---------------+------+------+--------+--------+--------+
| 50 | 3 | NULL | NULL | NULL | Miata | MS-001 | NULL |
+-----+------+---------------+------+------+--------+--------+--------+
| 51 | 3 | NULL | NULL | NULL | Miata | MS-002 | NULL |
+-----+------+---------------+------+------+--------+--------+--------+
=head2 advantages
Polymorphic retrieval costs exactly one SELECT, regardless of the
number of conforming types. Thus this mapping potentially is the most
efficient.
=head2 disadvantages
This mapping is very questionable according to relational
orthodoxy. Even if one decides to forgo these rules, using such a
mapping takes away many of the interesting features offered by modern
RDBM systems. Because nearly all the columns must allow NULL values,
we cannot take advantage of features like referential integrity
constraints, domain constraints, indexes, etc.
Also, as the table becomes cluttered with NULL values, the relative
number of significant columns in any given row tends towards zero: we
may end up retrieving rows consisting of a little information swimming
in a sea of NULLs.
In effect, this mapping may end up hindering performance instead of
improving it in presence of deep hierarchies with many attributes.
=head1 MAPPINGS SUPPORTED BY TANGRAM
Tangram supports both vertical mapping and filtered mapping, and any
hybrid of the two.
The 'table' attribute in the class description in the Schema can be
used to put the state of several classes on the same table. The table
name defaults to the class name, resulting in a vertical mapping.
For example, the following schema:
Tangram::Relational->schema( {
classes =>
[ Person =>
{
table => 'Persons',
fields => { string => [ qw( name ) ] }
},
NaturalPerson =>
{
table => 'Persons',
fields => { int => [ qw( age ) ] }
},
LegalPerson =>
{
table => 'Persons',
fields => { string => [ qw( form ) ] }
}
] } );
...specifies a pure filtered mapping for the Person hierarchy:
CREATE TABLE Persons
(
id INTEGER NOT NULL,
PRIMARY KEY( id ),
type INTEGER NOT NULL,
form VARCHAR(255) NULL,
age INT NULL,
name VARCHAR(255) NULL
);
The following schema:
Tangram::Relational->schema( {
classes =>
[ Person =>
{
table => 'Person',
fields => { string => [ qw( name ) ] }
},
NaturalPerson =>
{
table => 'NaturalPerson',
fields => { int => [ qw( age ) ] }
},
LegalPerson =>
{
table => 'Person',
fields => { string => [ qw( form ) ] }
}
] } );
...gives NaturalPerson its own table, but LegalPerson shares the
Person table:
CREATE TABLE Person
(
id INTEGER NOT NULL,
PRIMARY KEY( id ),
type INTEGER NOT NULL,
form VARCHAR(255) NULL,
name VARCHAR(255) NULL
);
CREATE TABLE NaturalPerson
(
id INTEGER NOT NULL,
PRIMARY KEY( id ),
type INTEGER NOT NULL,
age INT NULL
);