This document summarizes a lecture on database integrity and performance. It discusses various techniques for ensuring database integrity, including primary key constraints to prevent duplicate tuples, foreign key constraints to prevent dangling references, and attribute constraints to prevent inconsistent attribute values. It also covers views, which allow querying virtual tables, and indexes to improve query performance by enabling faster searching. The document proposes discussing index structures and report topics at the next meeting.

1. CS 542 Database Management SystemsControlling Database Integrity and PerformanceJ Singh January 31, 2011

2. Today’s TopicsDatabase IntegrityPrimary Key Constraints – Prevent DuplicatesForeign Key Constraints – Prevent Dangling ReferencesAttribute Constraints – Prevent Inconsistent Attribute ValuesTupleConstraints – More vigilant checking of attribute valuesAssertions – Paranoid integrity checkingViewsPerformance TopicsIndexesDiscussion of presentation topic proposals

3. Primary Key ConstraintsWhat are Primary Keys good for?Uniquely identify the subject of each tupleEnsure that there are no duplicatesCannot be null – that would imply a NULL subject.A table may not have more than one primary keyA Primary Key may consist of one or more columnsMultiple Unique keys are OKFor Table R, <P1, P2, …, Pm> together constitute a primary key if for each tuple in R,<P1, P2, …, Pm> are uniqueP1, P2, …, Pm are non-null<U1, U2, …, Um> together constitute a unique key if for each tuple in R,<U1, U2, …, Um> are uniqueBut U1, U2, …, Umcan be null

4. Foreign Key Constraints (p1)Main Idea: Prevent Dangling TuplesForeign KeyKey ReferenceForeign KeyMust point to a Key ReferenceCREATE TABLE City ( ::CountryCode char(3) REFERENCES Country(Code))Key ReferenceMust be unique or primary keyTry: INSERT INTO city(Name, CountryCode) value ('xyzzy', 'XYZ');Try: UPDATE cityset CountryCode='XYZ' where CountryCode='FIN';Key reference must already exist before a referencing tuple can be added

5. Foreign Key Constraints (p2)Alternative methods of defining a foreign keyCREATE TABLE City (CountryCode char(3) REFERENCES COUNTRY(Code), …)CREATE TABLE City (CountryCode char(3), …, FOREIGN KEY CountryCode [CONSTRAINT [ctyREFcntry]] REFERENCES COUNTRY(Code))CREATE TABLE City (CountryCode char(3), …) Then, later, ALTER TABLE City ADD [CONSTRAINT [ctyREFcntry]] FOREIGN KEY CountryCode REFERENCES COUNTRY(Code);Notation: [] signifies optional

6. Foreign Key Constraints (p3)Foreign KeyKey ReferenceReferential Integrity OptionsRestrict (default)Reject requestCascadeReflect changes backSet NullSet the foreign key to NULLChanges to Key ReferencesTry: DELETE FROM country WHERE code=‘FIN’;Try: UPDATE country SET Code='XYZ' WHERE Code='FIN‘;

7. Foreign Key Constraints (p4)Chicken and Egg definitionsCREATE TABLE chicken (cID INT PRIMARY KEY, eID INT REFERENCES egg(eID));CREATE TABLE egg(eID INT PRIMARY KEY,cID INT REFERENCES chicken(cID));Consistently failsCan’t define a foreign key to a table before it has been definedSolutionDefine the tables w/o constraintsCREATE TABLE chicken(cID INT PRIMARY KEY,eID INT); CREATE TABLE egg(eID INT PRIMARY KEY,cID INT);And then add foreign keysALTER TABLE chicken ADD CONSTRAINT c_e FOREIGN KEY (eID) REFERENCES egg(eID);ALTER TABLE egg ADD CONSTRAINT e_c FOREIGN KEY (cID) REFERENCES chicken(cID);

8. Foreign Key Constraints (p5)Chicken and Egg insertionINSERT INTO chicken VALUES(1, 1001);INSERT INTO egg VALUES(1001, 1);Still consistently failsNeed a way to postpone constraint checkingHow long to postpone?Until transaction commit SolutionDefine the tables with deferred constraint-checkingALTER TABLE chicken ADD CONSTRAINT c_e FOREIGN KEY (eID) REFERENCES egg(eID) INITIALLY DEFERRED DEFERRABLE;ALTER TABLE egg ADD CONSTRAINT e_c FOREIGN KEY (cID) REFERENCES chicken(cID) INITIALLY DEFERRED DEFERRABLE;And thenINSERT INTO chicken VALUES(1, 1001);INSERT INTO egg VALUES(1001, 1);COMMIT;

9. Attribute-Based ConstraintsNOT NULLThe most commonReasonability ConstraintsValidate incoming data? e.g.,Population Density < 30000Specification:Population INT(11) NOT NULL CHECK (Population <= 30000 * SurfaceArea),The condition in CHECK(cond) can take any value that a condition in WHERE(cond) can takeIncluding subqueriesThe attribute constraint is checked when assignedCan be violated underneath as long as it is not re-evaluatedFor example, if we update SurfaceArea, the violation won’t be flaggedNot implemented in all databases, e.g., MySQL

10. Tuple-Based ConstraintsValidate the entire tuple whenever anything in that tuple is updatedMore integrity enforcement than with attribute-based constraints e.g.,Population Density <= 30000Specification:Population INT(11) NOT NULL,CHECK (Population <= 30000 * SurfaceArea),The condition in CHECK(cond) can take any value that a condition in WHERE(cond) can takeIncluding subqueriesThe attribute constraint is checked when tuple is updatedIf we update SurfaceArea, the violation will be flaggedBut the violation ofCHECK (Population > ( SELECT SUM(Population) FROM City WHERE City.CountryCode = Code))which specifies a subquery involving another table, will not be flaggedNot implemented in all databases, e.g., MySQL

11. AssertionsValidate the entire database whenever anything in the database is updatedPart of the database, not any specific tableSpecification: Table-likeCREATE ASSERTION CountryPop CHECK ( NOT EXISTS (SELECT * FROM Country WHERE Population < (SELECT SUM(Population) FROM City WHERE City.CountryCode = Code)))Difficult to implement efficientlyOften not implementedI don’t know of any implementationsCan be implemented for specific cases using Triggers, see Section 7.5

12. ViewsAlso called Virtual ViewsDon’t actually exist in the database but behave as if they doCan be subsets of the data or joins – actually, arbitrary queriesSubset example,CREATE VIEW ct AS SELECT c.Name AS nm, c.countrycode AS cntryFROM city c WHERE population > 0Join exampleCREATE VIEW CityLanguage as SELECT city.name, city.countrycode, lang.languageas Language FROM city, countrylanguage as lang WHERE city.countrycode = lang.countrycode AND lang.isOfficial= ‘T‘;

13. Operations on Views (p1)SELECT SELECT * FROM CityLanguage WHERE Language='Dutch';Shouldn’t ‘temporarily’ create the table and SELECT from it.Should use the definition of CityLanguage to make a query, i.e.,SELECT * FROM (SELECT …blabla… FROM city, countrylanguage as lang WHERE city.countrycode = lang.countrycode AND lang.isOfficial = 'T') WHERE Language='Dutch';

14. Operations on Views (p2)UPDATE, INSERT not always possible, exceptCan sometimes be implemented using INSTEAD OF triggersModifications are permitted when the view is derived from a single table R andThe WHERE clause does not involve R in a SubqueryThe FROM clause can only consist of one occurrence of RThe valued of all attributes not specified in the view definition can be ‘manufactured’ by the databaseExample. For the view ctCREATE VIEW ct AS SELECT c.Name AS nm, c.countrycodeAS cntryFROM city c WHERE population > 0 the queryINSERT INTO ct (nm, cntry) values ('FirSPA', 'FIN') can be automatically rewritten as INSERT INTO CITY (Name, CountryCode) values ('FirSPA', 'FIN')

15. Top-Down Datalog Recursion RevisitedIDB’s are conceptualized (and implemented) as Viewsfor IDB predicate p(x,y, …) FOR EACH subgoal of p DO IF subgoal is IDB, recursive call; IF subgoal is EDB, look up

16. IndexesMain Idea: Data Structures for Fast SearchMotivation:Preventing the need for linear search through a big tableExample query: SELECT * FROM City WHERE CountryCode = 'FIN';Another: SELECT * FROM City WHERE Population > (0.4 * ( SELECT Population FROM Country WHERE CountryCode= Code));Expected time for first example: O(n). For the second, O(n2)DeclarationCREATE INDEX CityIndex ON City(CountryCode);CREATE INDEX CityPopIndex ON City(Population);CREATE INDEX CountryPopIndex ON Country(Population);

17. Selection of Indexes (p1)Why not create an index for every attribute?Useful indexes, and not so useful onesPrimary key?Unique key?From previous examples, CityIndex?CityPopIndex?CountryPopIndex?

18. Selection of Indexes (p2)The Mantra:Don’t define indexes too early: know your workload firstBe as empirical as is practicalThe Greedy approach to index selection:Start with no indexesEvaluate candidate indexes, choose the one potentially most effectiveRepeatQuery execution will take advantage of defined indexes

19. CS 542 Database Management SystemsReport ProposalsJ Singh January 31, 2011

20. Next meetingFebruary 7Index Structures, Chapter 14