Inferring the Principal Type and the Schema Requirements of an OQL Query

1
Inferring the Principal Type and the Schema
Requirements of an OQL Query
Greek State Scholarships Foundation
National Bank of Greece

• Agathoniki Trigoni and Gavin M. Bierman
• July 2001

2
Motivation

• What is a well-typed query?
• Queries addressed against multiple schemata
• Schema reconciliation interoperation
• Semi-structured data
• Generic queries and query definitions

DB/Schema A
DB/Schema B
Query Applications
Query
Query
Query
Query
DB/Schema C
Semi-Structured data
3
Objectives

• Inferring the
• Principal Type of an OQL Query
• Minimum Schema Type Requirements

define Dept_Managers (dept) as select e from
Employees e where e.positionmanager and
e.departmentdept select d from Departments
d where count (Dept_Managers (d)) gt 5
Employee Department department String
position Department
Employee int department String
position Departments List (int)
Employee String department String
position Department int id
4
Outline

• Motivation ? Schema Heterogeneity
• Objectives Type and Schema Inference of OQL
• Type System of OQL (ODMG 3.0)
• Type Compatibility / Resolving Constraints
• Typing Rules
• Inference Algorithm
• Conclusion

5
OQL Type System 1
Initial Type System
Subtyping Relation - Examples
6
OQL Type System 2
define q1 as select x from Students as x define
q2 as set ( first ( Students ) ) q1 union q2
Need for extended general types
q1 union q2
from Students
first (Students)
member type of any collection
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Extended Type System
any
nonfunctional
function
atomic
constructor
collection
boolean
orderable
class
struct
set/bag
list/array
string
int/float
char
set
bag
list
array
int
float
Generalisation Specialisation Relationship
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Type Compatibility - Constraints

• Expressive power of OQL
• Complex Type System

Variety of Type Constraints
If q1, q2, q3 have types
q3 q1 union q2
set (q1,q2)
q1 distinct(q2)
q1 q2.
q1 q2 5
etc
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Inference Rules
One rule per OQL construct hence
syntax driven inference algorithm
Type environment
Constraints
Examples
10
Resolving Constraints

• Constraint Resolution
• Gradual
• Accumulative

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Type inference Algorithm - Example
select c from Courses as c
where count(c.chapters) gt 10 and
first(c.chapters)Types
?3boolean
Member_Type(?1)?2
Constructor_Member_Type(?2,chapters) ?4
?4?collection(nonfunctional)
Member_Type(?4) ?5
string
?4 ?list/array(?5)
Equality_Compatible(?5,string)
?4 ?list/array(string)
?5 string
IT(q)bag(?2)
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Conclusion
OQL Expressiveness
Type Inference Model
Variety of Constraints
Complex Type System
Heterogeneity in DB Applications
Principal Query Type
Schema Requirements