Ahmad Ghulam Rasool

Data base indexing

Data Base Indexing• A database index is a data structure that

improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space to maintain the index data structure.

• Example :

How do database indexes work?

• Now, let’s say that we want to run a query to find all the details of any employees who are named ‘Jesus’? So, we decide to run a simple query like this:

• Example :• SELECT * FROM Employee WHERE Employee_Name = 'Jesus‘

What would happen without an index on the table?

• Well, the database software would literally have to look at every single row in the Customers table to see if the customerName for that row is ‘Jesus’

How a database index can help performance ?

• The whole point of having an index is to speed up search queries by essentially cutting down the number of records/rows in a table that need to be examined.

What kind of data structure is an index?

• B- trees are the most commonly used data structures for indexes.

• Time efficient • Deletions, and insertions can all be done

in logarithmic time• No need to sort data just use algorithm to

get sorted data like inorder,postorder etc• No need to search whole table

B-Tree

Another Data Structure is Hash Table

• Queries that compare for equality to a string can retrieve values very fast if they use a hash index.

• Example : SELECT * FROM Employee WHERE Employee_Name = ‘Jesus’

• Hash tables are not sorted data structures.• Suppose you want to find out all of the employees who

are less than 40 years old. How could you do that with a hash table index? Well, it’s not possible because a hash table is only good for looking up key value pairs.

Hash Table

What exactly is inside a database index?

• A database index does not store the values in the other columns of the same table.

• Example: Age and Employee_Address column values are not also stored in the index.

• An index stores a pointer to the table row.

How to create an index in SQL• Syntax: CREATE INDEX name_indexON

Employee (Employee_Name)

• How to create a multi-column index in SQL• Syntax: CREATE INDEX name_indexON

Employee (Employee_Name, Employee_Age)• ALTER TABLE tbl_name ADD PRIMARY KEY

(column_list): This statement adds a PRIMARY KEY, which means that indexed values must be unique and cannot be NULL

What is the cost of having a database index?

• It takes up space – and the larger your table, the larger your index.

• Whenever you add, delete, or update rows in the corresponding table, the same operations will have to be done to your index.

• Drop unused indexesSyntax: Alter table admin drop index searchAge

Index quries• SHOW INDEX FROM table_name• ALTER TABLE testalter_tbl ADD PRIMARY KEY (i);• ALTER TABLE testalter_tbl DROP PRIMARY KEY;• CREATE TABLE lookup( id INT NOT NULL, name CHAR(20), PRIMARY KEY USING BTREE (id))

Cont

• CREATE TABLE lookup( id INT NOT NULL, name CHAR(20), PRIMARY KEY USING HASH (id))

Thank you