1. Fabio Grandi Alma Mater Studiorum – Università di Bologna
A Relational Multi-Schema Data Model and Query Language for Full Support of Schema Versioning
Fabio Grandi Alma Mater Studiorum – Università di Bologna
SEBD 2002

2. Introduction Schema Evolution Schema Versioning
Automatic recovery of extant data after schema changes
Schema Versioning
Maintenance of past schemas (e.g. for legacy application support)
Two design options for the implementation of data repositories
SEBD 2002

3. Design Options for Extant Data
Single-pool solution Roddick, Roddick & Snodgrass [TSQL2 ’95]
All schema versions associated with a unique shared data repository
Multi-pool solution De Castro, Grandi & Scalas [TDB ’95, IS ’97]
Each schema version associated with a different private data repository
SEBD 2002

4. Single-pool - Example A 123 B A 125 B C
create schema SV1 create table R(A int, B int)
create schema SV2 alter table R add column C int A
123 B A
125 B C
SV1: R(A,B)
SV1: R(A,B) SV2: R(A,B,C)
SEBD 2002

5. Single-pool - Example (2)
create schema SV3 alter table R drop column B
create schema SV4 alter table R alter column A char A
125 B C A’
125 B C
A’’ xy
SV1: R(A,B) SV2: R(A,B,C) SV3: R(A,C)
SV1: R(A,B) <= R(A’,B) SV2: R(A,B,C) <= R(A’,B,C) SV3: R(A,C) <= R(A’,C) SV4: R(A,C) <= R(A’’,C)
SEBD 2002

6. Multi-pool - Example A 123 B A 125 B C 33 DP1: DP2: A 125 C 55 A xy C
SV1: R(A,B)
SV2: R(A,B,C) A
125 C 55 A xy C 44
DP3:
DP4:
SV3: R(A,C)
SV4: R(A,C)
SEBD 2002

7. Single-pool vs Multi-pool
Simple to implement with the completed schema solution
Reduces to a quite “trivial” view mechanism
Multi-pool:
More flexible and complex
Thus, potentially more useful, but…
Is it feasible ?
And what’s for ?
SEBD 2002

8. Multi-pool – what’s for?
From a conceptual perspective different schema versions correspond to different points of view
SV2
SV1
SV3
SV4
Application Domain
SEBD 2002

9. Schema Versions as Viewpoints
IT Market
US Market
EU Market
Different viewpoints on the same data involve
different structures at intensional level (schema)
different values at extensional level (data)
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10. A Concrete Example
The “Lark 2.5” car model in the global market ( diff. viewpoints involve diff. marketing strategies )
Lark K$
US Market
Lark K€ Euro4
EU Market
Lark GT 31 K€ Euro3
IT Market
SEBD 2002

11. A Multi-schema Query Language
Full potentialities of a multi-pool schema versioning approach can be exploited only by means of a query language which allows users/developers to express Multi-schema Queries that is involving data belonging to different data pools
MSQL - Multi-schema SQL extension
SEBD 2002

12. The MSQL Query Language
MSQL basic construct: contextualization of names and data references to schema versions
[SV: X] denotes the conceptual entity named “X” in schema version “SV”
SV: X denotes the extension wrt schema version “SV” of the conceptual entity “X”
the two mechanisms can be combined (e.g. as in SVi: [SVj: X] )
SEBD 2002

13. The MSQL Query Language (2)
Examples:
select * from [SVj: R] retrieve the table called R in SVj
select * from SVi: R retrieve the contents wrt SVi of R
select * from SVi: [SVj: R] retrieve the contents wrt SVi of the table called R in SVj
SEBD 2002

14. The MSQL Query Language (3)
Examples:
select [SVj: A] from R retrieve the column called A in SVj of R
select SVi: A from R retrieve the contents wrt SVi of R.A
select SVi: [SVj: A] from R retrieve the contents wrt SVi of the column called A in SVj of R
SEBD 2002

15. The MSQL Query Language (4)
Example:
select SVi: [SVj: A] from SVk: [SVl: R] retrieve the values wrt SVi of the column called A in SVj in the tuples belonging wrt SVk to the table called R in SVl
SEBD 2002

16. The MSQL Query Language (5)
Example: set schema USMKT; select NAME from EUMKT:CAR as EC, ITMKT:CAR as IC where IC.PRICE < EC.PRICE
retrieve the American names of all the cars also sold in Europe such that the Italian price is lower than the price applied in the rest of Europe
SEBD 2002

17. The Logical Storage Model
Provides a simple implementation scheme for a Multi-pool schema versioning database (on top of a traditional relational DBMS)
Is used to formally define the semantics of the MSQL language
Representation of the SVi data pool: RNamei(RID,Name) ANamei(AID,Name) RSchemai(RID,AID) RExti(RID,TID) AExti(TID,AID,Value)
SEBD 2002

18. Conclusions
Defense of the Multi-pool approach (from a conceptual design standpoint)
Introduction of the Multi-schema Query Language (MSQL)
Introduction of the Logical Storage Model
Future Work
development of implementation strategies
study of formal properties (e.g. correctness of a schema evolution process under the multi-pool approach)
SEBD 2002