Database Indexing 1

After this lecture, you should be able to:

Understand why we need database indexing.

Define indexes for your tables in MySQL.

See the performance improvement of Indexing over No-Indexing.

Database Indexing

Database Indexing 2

Employee (EID(auto-increment), Age, Salary)

Easy to retrieve employees by EID in increasing (or decreasing) order.

Difficult to retrieve all employees who are 55 years old (age) -> scan the entire table file.

One Example…

Solution: Create an index on age field -> the same idea with book index.

Database Indexing 3

Database indexing is a technique to help efficiently access a collection of records in multiple ways.

A database index is a data structure that improves the speed of operations on a database table.

Indexes can be created using one or more columns of a database table.

Note: in MySQL, a primary key column is automatically indexed for efficiency.

Database Indexing Overview

Database Indexing 4

Table is stored in a file. File of records is a collection of records

that may reside on several pages. Record id (rid) is sufficient to

physically locate record. Pages (Data Blocks) (4KB or 8KB) are

stored on disk. Indexes are data structures that allow us

to find the record ids of records with given values in index search key fields.

Data on External Storage

Database Indexing 5

Indexes: Data structures to organize records via trees or hashing. Like sorted files, they speed up

searches for a subset of records, based on values in certain (“search key”) fields

Updates are much faster than in sorted files.

Examples of indexing techniques: ordered files, B+ trees, hash based structures.

Typically, index contains auxiliary information (access method) that directs searches to the desired data entries.

Indexes

Database Indexing 6

Index Structure Examples

Index on the primary key field

Database Indexing 7

Index Structure Examples

A search tree

Database Indexing 8

Employee (EID, Age, Salary)

Difficult to retrieve all employees who are 55 years old (age) -> scan the entire table file.

Solution: Create an index on age field -> the same idea with book index.

Defining Index in MySQL

create index emp_age_idx using btree on Employee(Age);

drop index emp_age_idx on Employee;

Database Indexing 9

8 tabless_100_none s_100_btrees_1000_none s_1000_btrees_10000_none s_10000_btrees_100000_none s_100000_btree

Table s_10000_btree (sno,sname,status,city): contains 10000 records and have a B-tree index on sname field.

An experiment with database index

create index s_10000_btree_sname_idx using btree on s_10000_btree(sname);

Database Indexing 10

Populating data

$chars = "abcdefghijkmnopqrstuvwxyz023456789";for ($i = 0; $i < 10000; $i++){

$sname = substr(str_shuffle($chars), 0, 15);$status = rand(1, 2000);$city = substr(str_shuffle($chars), 0, 10);$query = "Insert into s_10000_btree

(sname, status, city) values ('$sname', $status, '$city')";

$result = mysql_query($query);echo "Executing: $query - result: $result</br>";

}

Database Indexing 11

Random Searches - Queries

select SQL_NO_CACHE *from s_100_nonewhere sname = 'abcdefghijkmnop‘;…select SQL_NO_CACHE *from s_100000_nonewhere sname = 'abcdefghijkmnop‘;

/* */

select SQL_NO_CACHE *from s_100_btreewhere sname = 'abcdefghijkmnop'

select SQL_NO_CACHE *from s_100000_btreewhere sname = 'abcdefghijkmnop’;

Database Indexing 12

Random Searches - Result

MyISAM - Random Search

0

0.005

0.01

0.015

0.02

0.025

0.03

Records#

None

btree

None 0.0005 0.0007 0.0029 0.0244

btree 0.0006 0.0006 0.0006 0.0006

100 1000 10000 1E+05

Database Indexing 13

Sequential Searches - Queries

…select SQL_NO_CACHE max(sname)from s_100000_nonewhere sname > 'abcdefghijkmnop' and sname < 'rmo8s5jh4ciyxfe';

/* */

…select SQL_NO_CACHE max(sname)from s_100000_btreewhere sname > 'abcdefghijkmnop' and sname < 'rmo8s5jh4ciyxfe';

Database Indexing 14

Sequential Searches - Result

MyISAM - Sequential Searches

0

0.01

0.02

0.03

0.04

0.05

Records#

None

btree

None 0.0006 0.0009 0.0047 0.0386

btree 0.0005 0.0005 0.0005 0.0005

100 1000 10000 1E+05