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Indexes

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What is an Index?

An index is a data structure used in database systems to perform faster lookup.

Indexes can greatly improve the performance of searches on the indexed column or columns.

Example: SELECT companyname

FROM supplier

WHERE supplierid = 'NBS'

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Indexes

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When to Create an Index?

Factors that can be considered to determine if you should create an index are as follows: • Keys and unique columns • Frequency of search • Size of table • Number of updates • Space considerations • Data distribution

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Indexes

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CREATE INDEX

The syntax is shown below:

CREATE [UNIQUE] INDEX <indexname> ON <table name>(<column list>)

This syntax creates an index on the

values of the attribute in <column list> from table <table name>.

Example: CREATE UNIQUE INDEX supplierindex ON suppliers(supplierid);

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DROP INDEX

The syntax is shown below:

DROP INDEX <index name>

This syntax removes an index

specified by the <index name>.

Example: DROP INDEX supplierindex;

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Indexes

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Composite Index

A composite index is an index that can contain one, two, or more columns. The syntax is shown below:

CREATE [UNIQUE] INDEX <indexname> ON <table name>(<column1>, <column1>, …<columnN>)

Example: CREATE INDEX name

ON student (stud_lname, stud_fname)

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Indexes

F0004

Composite Index

In creating composite index, the order of columns should be the same as the order of the columns in the table.

Composite indexes are useful for searching on all columns in the index or on the first columns only.

Example: CREATE INDEX fname_lname

ON student stud_fname, stud_lname;

CREATE INDEX lname_fname

ON student stud_lname, stud_fname;