Index Example Index Example

From Garcia-Molina, Ullman, and Widom: From Garcia-Molina, Ullman, and Widom:

Database Systems, the Complete BookDatabase Systems, the Complete Book

pp. 298 - 300pp. 298 - 300

What is an Index?What is an Index?

• Let’s say relation R has an attribute A• An index on A is a data structure that allows quick

access to tuples of R if you know the value of A• Implementation: hash table or similar data structure.

Indices and database designIndices and database design

• Important fact: disk accesses are typically the highest cost operation for a DBMS

• Having an index on A speeds up database lookups involving A

• However, it slows down insertions and deletions involving A, because the index must also be updated

Example from textbookExample from textbook

StarsIn(movieTitle, movieYear, StarName)• Query 1 (Q1): SELECT movieTitle, movieYear

FROM StarsIn

WHERE starName= s• Query 2 (Q2): SELECT starName

FROM StarsIn

WHERE movieTitle= t AND movieYear= y• Insertion (I): INSERT INTO StarsIn

SET StarName= s, movieTitle= t, movieYear= y

Assumptions: on average, each star has appeared in 3 movies, and each movie has 3 stars; table takes up 10 disk blocks

Cost of QueriesCost of Queries

Action No index Star index

Movie Index

Both indices

Q1 10 4 10 4

Q2 10 10 4 4

I 2 4 4 6

Conclusions:Conclusions:

• If lookups on an attribute A are much more common than insertions and deletions, it makes sense to add an index on A

• But if lookups are not common, the index may slow down database performance

ImplementationImplementation

• An index can be defined on multiple attributes A, B. In this case the domain is the set of ordered pairs (a, b) ε A x B

• Some DBMS implementers automatically add an index to each primary key attribute.

• This is useful because any insertion to a table with a key requires a lookup to ensure that the key remains unique.