Tangram::Relational::Mappings(3pm)			User Contributed Perl Documentation			Tangram::Relational::Mappings(3pm)

NAME

       Tangram::Relational::Mappings - Mapping inheritance

DESCRIPTION

       There are many ways of representing inheritance relationships in a relational database. This document describes three popular ways and how
       Tangram supports them.

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 strict disci-
       plines and 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 disadvan-
       tages.

       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	 |
		+---------------+		       +-----------------+

Horizontal Mapping
description
       Each 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 else-
       where (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 |
		+------+------+----------------+--------+

       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

       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.

Vertical Mapping
       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 Per-
       sons 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)

       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

       disadvantages

       The mapping potentially has the highest performance cost: it requires multiple SELECTs like the horizontal mapping, but in addition, these
       SELECTs use joins.

Filtered Mapping
       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	|
	  +-----+------+---------------+------+------+--------+--------+--------+

       advantages

       Polymorphic retrieval costs exactly one SELECT, regardless of the number of conforming types. Thus this mapping potentially is the most
       efficient.

       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.

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
	  );

perl v5.8.8							    2006-03-29					Tangram::Relational::Mappings(3pm)