System and Method for Efficient Relational Query Generation and Tuple-to-Object Translation in an Object-Relational Gateway Supporting Class Inheritance

1. A computer program product comprising a computer readable medium having computer program logic recorded thereon for enabling a processor to process object-oriented queries in a computing environment having a relational database management system (RDBMS), objects in said computing environment conforming to a class inheritance hierarchy defined by an object-oriented schema, said computer program logic comprising: receiving means for enabling the processor to receive an object-oriented query and at least one prefetch path from an object-oriented source, said object-oriented query identifying one or more target objects of a target class which are desired to be constructed, said at least one prefetch path identifying one or more prefetch path objects which are desired to be constructed; and query set generating means for enabling the processor to generate from said object-oriented query a set of relational queries that, when processed, enable said RDBMS to retrieve tuples required to initialize base attributes of said target objects that are defined by said target class and by any super-classes and sub-classes of said target class; wherein said query set generating means includes; (a) means for determining whether the number of super-classes of said target class is greater than a super-class limit, and (b) means for generating a query for each super-class of said target class if the number of super-classes of said target class is greater than said super-class limit, wherein said query, when processed, enables said RDBMS to retrieve base attributes of said each super-class.

2. The computer program product of claim 1, wherein said query set generating means comprises: means for enabling the processor to generate a translated object query from said object-oriented query, said translated object query being a relational database query that, when processed, enables said RDBMS to retrieve data needed to initialize base attributes of said target objects that are defined by said target class; and relational query generating means for enabling the processor to generate from said translated object query one or more relational queries that, when processed, enable said RDBMS to retrieve tuples required to initialize base attributes of said target objects that are defined by said target class and by any super-classes of said target class.

3. The computer program product of claim 2, wherein said object-oriented query includes a condition list, and wherein said RDBMS includes a first table that maps to said target class and zero or more second tables that respectively map to said any super-classes of said target class, wherein said relational query generating means comprises: means for enabling the processor to generate, from said translated object query, a relational query that, when processed by said RDBMS, enables said RDBMS to retrieve from said first table first tuples that satisfy said condition list, and that enables said RDBMS to retrieve from said second tables second tuples that have primary key values equal to primary key values of said first tuples.

4. The computer program product of claim 2, wherein said object-oriented query includes a condition list, and wherein said RDBMS includes a first table that maps to said target class and zero or more second tables that respectively map to said any super-classes of said target class, wherein said relational query generating means comprises: means for enabling the processor to generate a first relational query by modifying said translated object query to include a statement to order first tuples retrieved from said first table by a primary key of said target class; and means for enabling the processor to generate, from said translated object query, a second relational query for each super-class of said target class, said second relational query, when processed by said RDBMS, enabling said RDBMS to retrieve from a second table corresponding to said each super-class second tuples that satisfy said condition list, and that have primary key values equal to primary key values of said first tuples.

5. The computer program product of claim 1, wherein said query set generating means comprises: means for enabling the processor to generate a translated object query from said object-oriented query, said translated object query being a relational database query that, when processed, enables said RDBMS to retrieve data needed to initialize base attributes of said target objects that are defined by said target class; and relational query generating means for enabling the processor to generate from said translated object query one or more relational queries that, when processed, enable said RDBMS to retrieve tuples required to initialize base attributes of said target objects that are defined by said target class and by any sub-classes of said target class.

6. The computer program product of claim 5, wherein said object-oriented query includes a condition list, and wherein said RDBMS includes a first table that maps to said target class and zero or more second tables that respectively map to said any sub-classes of said target class, wherein said relational query generating means comprises: means for enabling the processor to generate a first relational query by modifying said translated object query to include a statement to order first tuples retrieved from said first table by a primary key of said target class; and means for enabling the processor to generate, from said translated object query, a second relational query for each sub-class of said target class, said second relational query, when processed by said RDBMS, enabling said RDBMS to retrieve from a second table corresponding to said each sub-class second tuples that satisfy said condition list, and that have primary key values equal to primary key values of said first tuples.

7. The computer program product of claim 1, wherein said object-oriented query includes a condition list, and wherein said RDBMS includes a first table that maps to said target class, zero or more second tables that respectively map to said any super-classes of said target class, and zero or more third tables that respectively map to said any sub-classes of said target class, wherein said query set generating means comprises: means for enabling the processor to generate a translated object query from said object-oriented query, said translated object query being a relational database query that, when processed, enables said RDBMS to retrieve data needed to initialize base attributes of said target objects that are defined by said target class; means for enabling the processor to generate a first relational query by modifying said translated object query to include a statement to order first tuples retrieved from said first table by a primary key of said target class; means for enabling the processor to generate, from said translated object query, a second relational query for each super-class of said target class, said second relational query, when processed by said RDBMS, enabling said RDBMS to retrieve from a second table corresponding to said each super-class second tuples that satisfy said condition list, and that have primary key values equal to primary key values of said first tuples; and means for enabling the processor to generate, from said translated object query, a third relational query for each sub-class of said target class, said third relational query, when processed by said RDBMS, enabling said RDBMS to retrieve from a third table corresponding to said each sub-class third tuples that satisfy said condition list, and that have primary key values equal to primary key values of said first tuples.

8. The computer program product of claim 1, said computer program logic further comprising: relational query set modifying means for enabling the processor to modify said set of relational queries according to said at least one prefetch path to produce a modified set of relational queries that, when processed, enable said RDBMS to retrieve tuples required to initialize base attributes of said prefetch path objects that are defined by said prefetch path class and by any super-classes and sub-classes of said prefetch path class.

9. The computer program product of claim 8, wherein said relational query set modifying means comprises: means for enabling the processor to identify a defining class in which each prefetch path is defined; means for enabling the processor to identify a query in said set of relational queries that is directed to retrieving tuples from said defining class; means for enabling the processor to modify said identified query so as to enable said RDBMS to retrieve foreign key information from a relational table corresponding to said defining class; and means for enabling the processor to generate an additional set of relational queries that, when processed, enables said RDBMS to retrieve tuples required to initialize base attributes of said prefetch path objects that are defined by said prefetch path class and by any super-classes and sub-classes of said prefetch path class.

10. The computer program product of claim 1, said computer program logic further comprising: translating means for enabling the processor to translate said tuples to object-oriented objects consistent with said class inheritance hierarchy.

11. The computer program product of claim 1, wherein said set of relational queries comprises a target tuple stream set and one or more sub-tuple stream sets, said target tuple stream set comprising queries that enable said RDBMS to retrieve tuples needed to initialize base attributes of said target objects defined by said target class and any super-classes of said target class, each sub-tuple stream set comprising queries that enable said RDBMS to retrieve information needed to initialize base attributes of said target objects defined by classes in a unique sub-class branch of said class inheritance hierarchy, said translating means comprising: means for enabling the processor to select a tuple retrieved as a result of processing queries in said target tuple stream set; means for enabling the processor to select a sub-tuple stream set; means for enabling the processor to select a greatest derived class in said selected sub-tuple stream set; means for enabling the processor to determine whether a primary key value of said selected tuple appears in a retrieved tuple that maps to said selected class; means for enabling the processor to create a new object of said selected class if a primary key value of said selected tuple appears in a retrieved tuple that maps to said selected class; and means for enabling the processor to initialize base attributes of said new object using information contained in said selected tuple and in other retrieved tuples having a primary key value equal to a primary key value of said selected tuple.

12. A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for processing object-oriented queries so as to retrieve data from a relational database management system (RDBMS), said method steps comprising: (1) receiving an object-oriented query and at least one prefetch path from an object-oriented source, said object-oriented query identifying one or more target objects of a target class which are desired to be constructed, said at least one prefetch path identifying one or more prefetch path objects which are desired to be constructed; and (2) generating from said object-oriented query a set of relational queries that, when processed, enable said RDBMS to retrieve tuples required to initialize base attributes of said target objects that are defined by said target class and by any super-classes and sub-classes of said target class; wherein step (2) includes the steps of; (a) determining whether the number of super-classes of said target class is greater than a super-class limit, and (b) if the number of super-classes of said target class is greater than said super-class limit, then generating a query for each super-class of said target class, wherein said query, when processed, enables said RDBMS to retrieve base attributes of said each super-class.

13. The program storage device of claim 12, wherein step (2) comprises the steps of: (a) generating a translated object query from said object-oriented query, said translated object query being a relational database query that, when processed, enables said RDBMS to retrieve data needed to initialize base attributes of said target objects that are defined by said target class; and (b) generating from said translated object query one or more relational queries that, when processed, enable said RDBMS to retrieve tuples required to initialize base attributes of said target objects that are defined by said target class and by any super-classes of said target class.

14. The program storage device of claim 13 in which said object-oriented query includes a condition list, and in which said RDBMS includes a first table that maps to said target class and zero or more second tables that respectively map to said any super-classes of said target class, wherein step (b) comprises the step of: generating, from said translated object query, a relational query that, when processed by said RDBMS, enables said RDBMS to retrieve from said first table first tuples that satisfy said condition list, and that enables said RDBMS to retrieve from said second tables second tuples that have primary key values equal to primary key values of said first tuples.

15. The program storage device of claim 13 in which said object-oriented query includes a condition list, and in which said RDBMS includes a first table that maps to said target class and zero or more second tables that respectively map to said any super-classes of said target class, wherein step (b) comprises the step of: generating a first relational query by modifying said translated object query to include a statement to order first tuples retrieved from said first table by a primary key of said target class; and generating, from said translated object query, a second relational query for each super-class of said target class, said second relational query, when processed by said RDBMS, enabling said RDBMS to retrieve from a second table corresponding to said each super-class second tuples that satisfy said condition list, and that have primary key values equal to primary key values of said first tuples.

16. The program storage device of claim 12, wherein step (2) comprises the steps of: (a) generating a translated object query from said object-oriented query, said translated object query being a relational database query that, when processed, enables said RDBMS to retrieve data needed to initialize base attributes of said target objects that are defined by said target class; and (b) generating from said translated object query one or more relational queries that, when processed, enable said RDBMS to retrieve tuples required to initialize base attributes of said target objects that are defined by said target class and by any sub-classes of said target class.

17. The program storage device of claim 16 in which said object-oriented query includes a condition list, and in which said RDBMS includes a first table that maps to said target class and zero or more second tables that respectively map to said any sub-classes of said target class, wherein step (b) comprises the step of: generating a first relational query by modifying said translated object query to include a statement to order first tuples retrieved from said first table by a primary key of said target class; and generating, from said translated object query, a second relational query for each sub-class of said target class, said second relational query, when processed by said RDBMS, enabling said RDBMS to retrieve from a second table corresponding to said each sub-class second tuples that satisfy said condition list, and that have primary key values equal to primary key values of said first tuples.

18. The program storage device of claim 12 in which said object-oriented query includes a condition list, and in which said RDBMS includes a first table that maps to said target class, zero or more second tables that respectively map to said any super-classes of said target class, and zero or more third tables that respectively map to said any sub-classes of said target class, wherein step (2) comprises the steps of: (a) generating a translated object query from said object-oriented query, said translated object query being a relational database query that, when processed, enables said RDBMS to retrieve data needed to initialize base attributes of said target objects that are defined by said target class; (b) generating a first relational query by modifying said translated object query to include a statement to order first tuples retrieved from said first table by a primary key of said target class; (c) generating, from said translated object query, a second relational query for each super-class of said target class, said second relational query, when processed by said RDBMS, enabling said RDBMS to retrieve from a second table corresponding to said each super-class second tuples that satisfy said condition list, and that have primary key values equal to primary key values of said first tuples; and (d) generating, from said translated object query, a third relational query for each sub-class of said target class, said third relational query, when processed by said RDBMS, enabling said RDBMS to retrieve from a third table corresponding to said each sub-class third tuples that satisfy said condition list, and that have primary key values equal to primary key values of said first tuples.

19. The program storage device of claim 12, said method steps further comprising the steps of: (3) modifying said set of relational queries according to said at least one prefetch path to produce a modified set of relational queries that, when processed, enable said RDBMS to retrieve tuples required to initialize base attributes of said prefetch path objects that are defined by said prefetch path class and by any super-classes and sub-classes of said prefetch path class; and (4) causing said RDBMS to process said set of relational queries.

20. The program storage device of claim 19, wherein step (3) comprises the steps of: identifying a defining class in which each prefetch path is defined; identifying a query in said set of relational queries that is directed to retrieving tuples from said defining class; modifying said identified query so as to enable said RDBMS to retrieve foreign key information from a relational table corresponding to said defining class; generating an additional set of relational queries that, when processed, enables said RDBMS to retrieve tuples required to initialize base attributes of said prefetch path objects that are defined by said prefetch path class and by any super-classes and sub-classes of said prefetch path class.

21. The program storage device of claim 12, wherein said method steps further comprise the steps of: (3) translating said tuples to object-oriented objects consistent with a class inheritance hierarchy defined by an object-oriented schema.

22. The program storage device of claim 21 in which said set of relational queries comprises a target tuple stream set and one or more sub-tuple stream sets, said target tuple stream set comprising queries that enable said RDBMS to retrieve tuples needed to initialize base attributes of said target objects defined by said target class and any super-classes of said target class, each sub-tuple stream set comprising queries that enable said RDBMS to retrieve information needed to initialize base attributes of said target objects defined by classes in a unique sub-class branch of said class inheritance hierarchy, step (3) comprising the steps of: (a) selecting a tuple retrieved as a result of processing queries in said target tuple stream set; (b) selecting a sub-tuple stream set; (c) selecting a greatest derived class in said selected sub-tuple stream set; (d) determining whether a primary key value of said selected tuple appears in a retrieved tuple that maps to said selected class; (e) if a primary key value of said selected tuple appears in a retrieved tuple that maps to said selected class, then creating a new object of said selected class; and (f) initializing base attributes of said new object using information contained in said selected tuple and in other retrieved tuples having a primary key value equal to a primary key value of said selected tuple.

23. The program storage device of claim 22, wherein step (3) further comprises the following steps that are performed if it is determined in step (d) that a primary key value of said selected tuple does not appear in a retrieved tuple that maps to said selected class: (g) determining whether there are additional classes in said selected sub-tuple stream set left to process; (h) if there are additional classes in said selected sub-tuple stream set left to process, then selecting a next most derived class in said selected sub-tuple stream set; and (i) returning to step (d).

24. The program storage device of claim 23, wherein step (3) further comprises the following steps that are performed if it is determined in step (g) that there are no additional classes in said selected sub-tuple stream set left to process: (j) determining whether there are additional sub-tuple stream sets left to process; (k) if there are additional sub-tuple stream sets left to process, then selecting another of said sub-tuple stream sets; and (m) returning to step (c).

25. The program storage device of claim 24, wherein step (3) further comprises the following steps that are performed if it is determined in step (j) that there are no additional sub-tuple stream sets left to process: (n) creating a new object of said target class; and (o) initializing base attributes of said new object of said target class using information contained in said selected tuple and in other retrieved tuples having a primary key value equal to a primary key value of said selected tuple.

26. A computer program product comprising a computer readable medium having computer program logic recorded thereon for enabling a processor in a computer system to process object-oriented queries so as to retrieve data from a relational database management system (RDBMS), said computer program product comprising: a query generator to enable said processor to generate from an object-oriented query a set of relational queries, said object-oriented query identifying one or more target objects of a target class which are desired to be constructed, said set of relational queries, when processed, enabling said RDBMS to retrieve tuples required to initialize base attributes of said target objects that are defined by said target class and by any super-classes and sub-classes of said target class, wherein each of said relational queries is designed to retrieve tuples from fewer than a predetermined number of tables in said RDBMS; and a tuple-to-object translator to enable said processor to translate said tuples to object-oriented objects consistent with a class inheritance hierarchy defined by an object-oriented schema.

27. The computer program product of claim 26, wherein said query generator determines whether the number of super-classes of said target class is greater than a super-class limit, and generates a query for each super class of said target class if the number of super-classes of said target class is greater than said super-class limit, wherein said query, when processed, enables said RDBMS to retrieve base attributes of said each super-class.