views - courses.cs.vt.educourses.cs.vt.edu/~cs4604/fall09/lectures/lecture-08-views.pdf · views...

Views Materialised Views

T. M. Murali

September 21, 2009

T. M. Murali September 21, 2009 CS 4604: Views

What are Views?

I A view is a relation that does not exist physically.

I A view is defined by a query over other relations (tables and/or views).

I Just like a table, a view can be queried: the query processor replacesthe view by its definition.

I Just like a table, a view can be used in other queries.

I Unlike a table, a view cannot be updated unless it satisfies certainconditions.

Defining a View

I CREATE VIEW ViewName AS Query;

I Suppose we want to perform a set of queries on those students whohave taken courses in the computer science departments.

I Let us create a view to store the PIDs of these students and thenumbers of the CS courses they are taking.

CREATE VIEW CSStudents

(PID, CSCourseNumber)

SELECT StudentPID, NumberFROM TakeWHERE (DeptName = ’CS’);

I Can name attributes in the view’s schema.

Defining a View

CREATE VIEW CSStudents(PID, CSCourseNumber) AS

Querying Views

I We can query views just as we can query tables.

I How many students took a CS course, i.e., what was the totalenrollment in CS courses?

SELECT COUNT (PID)

FROM CSStudents;

I Interpreting this query: replace view by its definition.

SELECT COUNT (StudentPID)

FROM(SELECT StudentPID, NumberFROM TakeWHERE (DeptName = ’CS’);

Querying Views

SELECT COUNT (PID)

FROM CSStudents;

Querying Views

SELECT COUNT (PID)

FROM CSStudents;

Querying Views

SELECT COUNT (PID)

FROM CSStudents;

Modifying Views

I What does it mean to modify a view?

I How is tuple deletion from a view executed?

I Can we insert a tuple into a view? Where will it be inserted, since aview does not physically exist?

I Can we insert tuples into any view?

SQL includes rules that specifywhich views are updatable.

Modifying Views

I What does it mean to modify a view?

I How is tuple deletion from a view executed?

I Can we insert a tuple into a view? Where will it be inserted, since aview does not physically exist?

I Can we insert tuples into any view? SQL includes rules that specifywhich views are updatable.

Deleting Views

I DROP VIEW CSStudents;

I Only view definition deleted.

I Underlying tables are not affected.

Deleting Tuples from CSStudents

I Delete tuples for students taking ’CS 4604’.

DELETE FROM CSStudents

WHERE (CSCourseNumber = 4604);

I Deletion is executed as if were executing

DELETE FROM Take

WHERE (Number = 4604);

I Incorrect: non-CS tuples where (Number = 4604) will be deleted.

I Correct deletion adds condition in the WHERE clause of the viewdefinition to the WHERE clause of the DELETE FROM statement.

WHERE (CSCourseNumber = 4604) AND (DeptName = ’CS’);

DELETE FROM Take

Inserting a Tuple into CSStudents

INSERT INTO CSStudents

VALUES (’123-45-6789’, 4604);

I Insertion is executed as if we were executing

INSERT INTO Take(StudentPID, Number)

VALUES (’123-45-6789’, 4604);I Two problems:

1. Key for Take is (PID, DeptName, Number). What value does thenew tuple get for DeptName?

2. Even if we had not declared key for Take, new tuple gets NULL valuefor DeptName ⇒ it is not a part of the view after insertion!

VALUES (’123-45-6789’, 4604);

I Two problems:

VALUES (’123-45-6789’, 4604);

VALUES (’123-45-6789’, 4604);I Two problems:

Correcting the Definition of CSStudents

I Include DeptName in the view’s schema.

CREATE VIEW CSStudents ASSELECT StudentPID, DeptName, NumberFROM TakeWHERE (DeptName = ’CS’);

I Write INSERT INTO as follows:

VALUES (’123-45-6789’, ’CS’, 4604);

Correcting the Definition of CSStudents

I Include DeptName in the view’s schema.

CREATE VIEW CSStudents ASSELECT StudentPID, DeptName, NumberFROM TakeWHERE (DeptName = ’CS’);

I Write INSERT INTO as follows:

VALUES (’123-45-6789’, ’CS’, 4604);

Updatable Views

I SQL has complex rules for when a view can be modified.

I Defined by selecting some attributes from one relation R.

I R may itself be an updatable view.

I Use SELECT and not SELECT DISTINCT.

I WHERE clause must not involve R in a sub-query.

I FROM clause can contain only one occurrence of R and must notcontain any other relation.

I SELECT clause must contain enough attributes so that for every tupleinserted into the view, other attributes can get NULL values or defaultvalues.

I An attribute that is declared NOT NULL and has no default must bementioned in the SELECT clause.

Materializing Views

I Some views may be frequently used in queries.

I It may be efficient to materialise such a view, i.e., maintain its valueat all times as a physical table.

I Cost of materialising views:

I Recomputing it when the underlying tables change.I Materialised view may be much larger than original relations, e.g., in

the case of joins.

Materializing Views

I Some views may be frequently used in queries.

I It may be efficient to materialise such a view, i.e., maintain its valueat all times as a physical table.

I Cost of materialising views:I Recomputing it when the underlying tables change.I Materialised view may be much larger than original relations, e.g., in

the case of joins.

Maintaining Materialised Views

CREATE MATERIALIZED VIEW CSStudents AS

SELECT StudentPID, DeptName, Number

FROM Take

WHERE (DeptName = ’CS’);

I When does CSStudents need to be maintained?

Insertion/deletion/update of Take.I How expensive is the modification of CSStudents?

I Insertion of tuple: Insert tuple into CSStudents only if new tuple hasDeptName = ’CS’.

I Deletion of tuple: same reasoning as insertion.I Update of tuple: View as delete followed by insert.

I Key idea is that many materialised views can be updatedincrementally.

I Incremental maintenance of a view that involves a join: read Chapter8.5.1 of the textbook.

FROM Take

I When does CSStudents need to be maintained?Insertion/deletion/update of Take.

I How expensive is the modification of CSStudents?

I Insertion of tuple: Insert tuple into CSStudents only if new tuple hasDeptName = ’CS’.

FROM Take

I How expensive is the modification of CSStudents?I Insertion of tuple: Insert tuple into CSStudents only if new tuple has

DeptName = ’CS’.

FROM Take

DeptName = ’CS’.I Deletion of tuple: same reasoning as insertion.I Update of tuple: View as delete followed by insert.

FROM Take

DeptName = ’CS’.I Deletion of tuple: same reasoning as insertion.I Update of tuple: View as delete followed by insert.

Periodic Maintenance

I Consider a database that records the inventory of a department store.

I A view may aggregate buyer patterns for further analysis.

I A (materialised) view is a good candidate for storing these aggregateddata.

I Since such analysis is performed periodically, materialised view neednot be maintained after each customer transaction. It can be updateat regular intervals, for instance once every day.

I Automatic creation of materialised views: Read Chapter 8.5.4 of thetextbook.

Periodic Maintenance

I Consider a database that records the inventory of a department store.

I A view may aggregate buyer patterns for further analysis.

I A (materialised) view is a good candidate for storing these aggregateddata.

I Since such analysis is performed periodically, materialised view neednot be maintained after each customer transaction. It can be updateat regular intervals, for instance once every day.

I Automatic creation of materialised views: Read Chapter 8.5.4 of thetextbook.

Rewriting Queries Using Materialised Views

I In practice, views are materialised because they are helpful to answercommon queries.

I Can we rewrite a query to use a materialised view rather than theoriginal relations?

I Find names and addresses of students taking CS courses?

SELECT Name, AddressFROM Students, TakeWHERE (Students.PID = Take.StudentPID) AND (DeptName =’CS’);

I Can we rewrite the query using the view CSStudents?

SELECT Name, AddressFROM Students, CSStudentsWHERE (Students.PID = CSStudents.StudentPID);

Rules for Rewriting Queries

I Complete sets of rules is very complex. We discuss a simple rule torewrite a query using a materialised view.

View V :SELECT LV

FROM RV

WHERE CV .

Query Q:SELECT LQ

FROM RQ

WHERE CQ .

New query Q ′:SELECT LQ

FROM V , RQ − Rv

WHERE C .

I We can replace Q by the new query Q ′ ifI RV ⊆ RQ .I CQ ≡ CV AND C , for some condition C , which may be empty.I If C is not empty, then attributes of relations in RV that C mentions

are also in LV .I Attributes in LQ that come from relations in RV are also in the list of

attributes LV .

Rules for Rewriting Queries

I Complete sets of rules is very complex. We discuss a simple rule torewrite a query using a materialised view.

View V :SELECT LV

FROM RV

WHERE CV .

Query Q:SELECT LQ

FROM RQ

WHERE CQ .

New query Q ′:SELECT LQ

FROM V , RQ − Rv

WHERE C .

I We can replace Q by the new query Q ′ ifI RV ⊆ RQ .I CQ ≡ CV AND C , for some condition C , which may be empty.I If C is not empty, then attributes of relations in RV that C mentions

are also in LV .I Attributes in LQ that come from relations in RV are also in the list of

attributes LV .

views - courses.cs.vt.educourses.cs.vt.edu/~cs4604/fall09/lectures/lecture-08-views.pdf · views...

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