Some High Level Language Some High Level Language Constructs for Data of Type Constructs for Data of Type

RelationRelation

Author: Author: Joachim W. Schmidt (Hamburg Joachim W. Schmidt (Hamburg university, west Germany) university, west Germany) From ACM Transactions on Database From ACM Transactions on Database Systems, September 1977 Systems, September 1977

Presented by: Presented by: Shnaiderman LilaShnaiderman Lila

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• Fact: (almost) All Programs Use Persistent Data • File Based Approach: open, close, get, put,

printf, etc. – Pain in the neck!

• Relational Algebra:– Traditional: invoke SQL from Pascal/C/ …

• Cumbersome.• Not type safe, checked only at runtime.• DB types may be different from Pascal types.

• This Work: Language mechanisms for– Data Query (a la relational algebra).– Data Modification (insert, delete, etc).

• missing in Relational Algebra.

MotivationMotivation

Reminder of RA•Selection (σ) - Selects a subset of rows•Cartesian-product (X) Allows us to combine two relations.•Projection (π) - Deletes unwanted columns from relation.•Set-difference (-) Tuples in R1, but not in R2.•Union (U) Tuples in R1 or in R2.•Renaming (ρ) – renames a column in a relation

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OutlineOutline• To propose language constructs for

performing database operations in a declarative manner:– A repetition statement controlled by

relations.– Predicates as a generalization of

Boolean expressions.– A constructor for relations using

predicates.

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Main ConceptsMain Concepts• Tuple - implemented as a record with

fields of type string or scalar.• Relation - a set of tuples, while some of

the fields of the relation serves as a key• Key – Uniquely determine a tuple in a

relation. • Key Order: automatically defined

– Natural order of each primitive type (Integer, String)

– Lexicographic order if multiple keys.

Example: for key <City,Price>

Jerusalem, $12Jerusalem, $13Jerusalem, $14Tel-Aviv, $11

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Defining a Relation in Pascal-RDefining a Relation in Pascal-R

• Example: instructors are characterized by the attributes: ID (unique), status and name:type iRecord = record ID, Status : integer;

Name : string end;

iRelation = relation <ID> of iRecord;

var instructors : iRelation;relation key

record type

Just like Pascal

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Elementary Operators on RelationsElementary Operators on Relations

• Elementary Altering Operations:– Insertion operator: rel1 :+ rel2;– Deletion operator: rel1 :- rel2;– Replacement operator: rel1 :& rel2;– Assignment operator: rel1 := rel2;

Result: rel1

ID Status Name

111 1 Moshe

222 2 Galit

ID Status Name

111 1 Bracha

222 2 Galit

333 1 Michal

ID Status Name

111 1 Moshe

222 2 Galit

333 1 Michal

Example:rel1 rel2

ID Status Name

111 1 Moshe

ID Status Name

111 1 Bracha

222 2 Galit

ID Status Name

111 1 Bracha

222 2 Galit

333 1 Michal

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Similarity to Sets in PascalSimilarity to Sets in Pascal• Pascal sets operations:

– Equivalents to operations on relations:• S1 := S2 Assign a set to a set variable. • S1 + S2 Union of sets. • S1 - S2 Difference between two sets.

– Other: Intersection, Symmetric difference, comparison operators, membership test

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Literals of Type RelationLiterals of Type Relation

• An elementary Relation Constructor– Constructed from a record variable :

‘rel := [rec]’– Empty relation : ‘[]’.

Example:

var instructors : iRelation; i : iRecord;

begin

…

i.ID := 222;

i.Status := 1;

i.Name := ‘Michal’

instructors := [i] end.

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Bad Old File-Like Operations on RelationsBad Old File-Like Operations on Relations

• Elementary Retrieval Operation – allows tuple-wise reading of relation variables. Defined with:– rel↑ - buffer variable for the current tuple.– low(rel) –updates the buffer with the

tuple with the lowest key value.– Next(rel) – updates the buffer with the

tuple with the next highest key value.– aor(rel) – returns true if all the tuples

already passed else false.

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ImplementationImplementationprogram DBuser(DB)

type

…

iRecord = … ;

iRelation = … ;

var

DB:database instructors:iRelation; … end;

result1, … : iRelation;

result7 : relation <rname, rtitle> of record … end;

begin with DB do

…

end.•Compiler was modified to accept the syntax•Run time library was added to handle the execution of the new constructs•Database counts as an external variable

Similar to PASCAL filesCan be connected to via a parameter in the program header

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Elementary Retrieval exampleElementary Retrieval example

• Find instructors with status = 2 (senior lecturer).

result = [];low(instructors);while not aor(instructors) dobegin if instructors↑. Status = 2 then result :+ [instructors↑];next(instructors)

end

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Problems with the old file model Problems with the old file model

• The ordering of tuple access is irrelevant– Almost no possibilities for automatic optimization.

• The language construct for relation is inadequate– File concept

• Only one global file pointer per relation• The notation can be more declarative.

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• Example: Find instructors with Status = 2 (senior lecturer).

result = [];low(instructors);while not aor(instructors) dobegin

if instructors↑.Status = 2 then result :+ [instructors↑];next(instructors)

end.

The Repetition Statement The Repetition Statement foreachforeach

• Example: Find instructors with Status = 2 (senior lecturer).

result = [];

foreach i in instructors do

if i.Status = 2

then result :+ [i]

Control record variable

Range relation variable

statement

ForeachForeach in Java 5 in Java 5

void printAll(Collection<instructor> c) { for (instructor i : c)

i.print(); } foreach i in instructors

for instructor i c:

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Nested Nested foreachforeach• Example : Find all those instructors who has lecture

on Fridaytype …

tRecord = record ID, CourseID, Time : integer; Day, Room : string end;

tRelation = relation < ID, CourseID, Day > of tRecord; var instructors,result : iRelation;

timetable : tRelation;begin

…result = [];foreach i in instructors doforeach t in timetable do

if (e.ID = t.ID) and (t.Day = ‘Friday’) then result :+ [i]

end.

• The inner loop traversed too often…• Redundant operations performed in runtime…• The inner loop used only as test condition.

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foreachforeach as as ∃∃ logical condition logical condition • Example: Find all those instructors whose

lecture of Fridayvar …

hasLecOnFriday : boolean;begin

…result = [];foreach i in instructors dobegin hasLecOnFriday := false;foreach t in timetable do

hasLecOnFriday := hasLecOnFriday or (i.ID = t.ID) and (t.Day = ‘Friday’)if hasLecOnFriday then result :+ [i]end

end.

The logical condition can be replaced with:

some rec in rel (<logical expression>)

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foreachforeach as as ∀∀ logical condition logical condition• Example: Find all instructors who give no

lecture.

var … hasNoLecture : boolean;

begin … result = []; foreach i in instructors do begin hasNoLecture := true;

foreach t in timetable do hasNoLecture := hasNoLecture and

(i.ID ≠ t.ID);if hasNoLecture then result :+ [i]end

end.

The logical condition can be replaced with:

all rec in rel (<logical expression>)

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Extending Pascal with Extending Pascal with advanced logic expressions advanced logic expressions

• In “regular Pascal”:– A and B– A or B– Not A

• Pascal-R has First Order Predicate Logic– ∃xs.p(x)– ∀ xs.p(x)

Lets rewrite the previous examples

Predicate calculus

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some some PredicatePredicate

• Example : Find all those instructors whose lecture is on Friday

result = [];

foreach i in instructors do

if some t in timetable

((i.ID = t.ID) and

(t.Day = ‘Friday’))

then result :+ [i]

•“while not aor(timetable) do” – sequential tuple-wise processing ordered by key values •“foreach t in timetable do” – still tuple-wise and sequentially but with no specific order•“some t in timetable” - Gives possibility for efficient implementation (processing tuple sets in parallel)

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AllAll predicate predicate

•Example: Find all instructors who give no lecture.

result = [];foreach i in instructors do

if all t in timetable(t.ID ≠ t.instructorID);

then result :+ [i]

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Nested predicatesNested predicates• Example: Find those instructors who give no

lectures above the first year (courses with level 1)

result = [];

foreach i in instructors do

if all t in timetable ((i.ID ≠ t.instructorID) or

some c in courses

((t.CourseID = c.ID) and (c.Level = 1)))

then result :+ [i]

end.

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Generalizing the Relation ConstructorGeneralizing the Relation Constructor

• So far all literals have one or zero records

• Construction of bigger relations – by adding one record at a time. – Sequential process…

• Let’s use the constructs we already created:res = [each i in instructors : i.Status = 2]

This is like ‘SELECT’ statement in SQL:SELECT * FROM instructors WHERE Status = 2;

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General Relation Constructor examplesGeneral Relation Constructor examples

var … ,res3,res4,res5,res6 : iRelation;begin…

res3 := [each i in instructors : i.Status = 2];

res4 := [ each i in instructors : some t in timetable ((i.ID = t.instructorID)

and (t.Day = ‘Friday’))];

res5 := [each i in instructors : all t in timetable (i.ID ≠ t.instructorID)];

res6 := [each i in instructors : all t in timetable ((i.ID ≠ t.instructorID)

or some c in courses ((t.CouseID = c.ID) and (c.cLevel = 1)))]

end.

SELECT * FROM instructors WHERE Status = 2;

SELECT * FROM instructors i, timetable t WHERE (i.ID = t.instructorID) AND (t.Day = ‘Friday’);

Structural equivalence Problem•Pascal uses name equivalence method.•The assignment of “each” constructor into the target relation requires structural equivalence method…•Requires changes that contradict the spirit of Pascal…

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Generalizing the Relation Constructor Generalizing the Relation Constructor (continue)(continue)

• Example: For those instructors who give lectures, find the names and the title and year of their publications.

res : relation <rname, rtitle> of record rname, rtitle : string ryear : integer

end;begin…res := [each(i.Name; p.title; p.year)

for i, p in instructors, papers : (i.ID = p.instructorID) and some t in timetable

((i.ID = t.instructorID)]end.

SELECT i.Name, p.title, p.yearFROM instructors i, papers p, timetable t WHERE (i.ID = p.instructorID) and

(i.ID = t.instructorID)]

•Advantages of this constructor:•Declarative• similar to RA• opens the door for optimizations.

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Additional topics of interestAdditional topics of interest• Unsolved issues:

– Tools for altering data consistently– Taking care of simultaneous processing of a database by

several users– Security issues– Error handling

•

• Similar modification in many modern languages (like we will see in the continuation of the course)

1970 19781979/

19831988

Codd relational Model Pascal-R

PLAIN RIGEL

Adaplex

SQL

Pascal-R development

DBPL

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ConclusionConclusion

• What was achievedDeclarative!Leaves a lot of space for optimization!Includes Powerful retrieval facilities on relations (tuples sets level)!The expressive power of the proposed constructs is satisfactory!Type checking is possible, but requires the database schema to be known at compile time and remain unchanged in runtime…Contradicts the spirit of Pascal…