What is indexing in SQL? Meredith India interview question
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Answers ( 12 )
Index is a special type of object which is created on columns of table. One table can contain one or more indexes. Index can be used on operation SELECT, DELETE, UPDATE to increase the performance of large amount of Data.
Syntax : CREATE [UNIQUE] INDEX ON
An index can be used to efficiently find all rows matching some column in your query and
then walk through only that subset of the table to find exact matches. If you don’t have
indexes on any column in the WHERE clause, the SQL server has to walk through the whole table
and check every row to see if it matches, which may be a slow operation on big tables.
Creating an index involves the CREATE INDEX statement, which allows you to name the index,
to specify the table and which column or columns to index, and to indicate whether the index
is in an ascending or descending order.
Basic syntax
CREATE INDEX index_name ON table_name;
Single Column Index
CREATE INDEX index_name
ON table_name (column_name);
Unique Index
CREATE UNIQUE INDEX index_name
on table_name (column_name);
The indexes are special objects which built on top of tables. The indexes can do an operation like SELECT, DELETE and UPDATE statement faster to manipulate a large amount of data. An INDEX can also be called a table and it has a data structure. An INDEX is created on columns of a table. One table may contain one or more INDEX tables.
Code: CREATE [UNIQUE] INDEX ON
Functions of Indexes:
INDEXES can locate information within a database very fast.
An INDEX makes a catalog of rows of a database table as row can be pointed within a fraction of the time with a minimum effort.
A table INDEX is a database structure that arranges the values of one or more columns in a specific order.
The performance of an INDEX can not be recognized much when dealing with relatively small tables.
INDEX can work properly and quickly for the columns that have many different values.
It takes a long time to find information for one or a combination of columns from a table when there are thousands of records in the table. In that case, if indexes are created on that column, which is accessed frequently, the information can be retrieved quickly.
The INDEX first sorts the data and then it assigns an identification for each row.
The INDEX table having only two columns, one is a row id and another is indexed-column (ordered).
When data is retrieved from a database table based on the indexed column, the index pointer searches the rowid and quickly locates that position.in the actual table and display, the rows sought for.
We use CREATE INDEX to create indexes in SQL.
where INDEXES are used to retrieve data from DATABASE more quickly.
They are just used to speed up the queries, searches.
example:
CREATE INDEX index_name
on TABLE_NAME (Column1, Column2…);
CREATE INDEX idx_name
ON Student (FirstName, LastName);
We use CREATE INDEX to create indexes in SQL.
where INDEXES are used to retrieve data from DATABASE more quickly.
They are just used to speed up the queries, searches.
example:
CREATE INDEX index_name
on TABLE_NAME (Column1, Column2…);
CREATE INDEX idx_name
ON Student (FirstName, LastName);
Indexes are used by queries to find data quickly from tables. Indexes are created on tables and views. Index on a table or a view is similar to that of a book.
If you do not have an index and I ask you to locate a specific chapter in the book, then, you will have to look at every page of the book. On the other hand if you have an index then, you will look at the page number and directly go to that page number. It helps you to save time drastically.
In a similar way, Table and view indexes can help to find the data quickly. It improves the performance of the query. If there is no index then, query engine will check each and every row from the beginning to the very end. This is called Table Scan which is bad for performance.
For ex: we have an Employee table which contains columns namely Id, name, salary and gender.
id name salary gender
1 sam 2500 male
2 pam 6500 male
3 john 4500 male
4 sara 5500 female
5 ted 3500 female
Query is to find out employees who have salary > 5000 and salary < 7000.
Since there is no index on salary now, table scan will take place which will have a bad effect on the performance.
In order to overcome this, index is used.
CREATE INDEX IX_Employee_salary
on Employee (Salary ASC)
This will create an index on the salary column of the Employee table.
Now, salaries between 5000 and 7000 will be there in the bottom of the table. SQL will directly fetch the row addresses from the index and directly fetch the records from the table, rather than scanning each and every row. This is known as Index seek.
1) Indexes are special lookup tables that the database search engine can use to speed up data retrieval by using a pointer. It can reduce disk I/O(input/output) by using a rapid path access method to locate data quickly. Simply put, an index is a pointer to data in a table. An index in a database is very similar to an index in a book
2) An index helps to speed up SELECT queries and WHERE clauses, but it slows down data input, with the UPDATE and the INSERT statements as indexes need to be updated as well
3) Indexes can be created or dropped with no effect on the data
4) The basic syntax of a CREATE INDEX and DROP INDEX is:
CREATE INDEX index_name ON table_name;
DROP INDEX tablename.index_name; (SQL Server)
ALTER TABLE table_name DROP INDEX index_name; (MySQL)
5) Unique indexes: Multiple values not allowed to enter into the table
CREATE UNIQUE INDEX index_name ON table_name (column_name);
6) Single-column index: one table column. A composite index: on two or more columns of a table. This choice depends on the number of columns used in the WHERE clause
CREATE INDEX index_name ON table_name (column_name);
CREATE INDEX index_name ON table_name (column1, column2);
7) Implicit indexes: automatically created by the database server when an object is created. Indexes are automatically created for primary key constraints and unique constrain
8) When should indexes be created –
a) A column contains a wide range of values. b) A column does not contain a large number of null values
c) One or more columns are frequently used together in a where clause or a join condition
9) When should indexes be avoided –
a) The table is small b) The columns are not often used as a condition in the query
c) The column is updated frequently
The indexes are special objects which built on top of tables. The indexes can do an operation like SELECT, DELETE, and UPDATE statements faster to manipulate a large amount of data. An INDEX can also be called a table and it has a data structure. An INDEX is created on columns of a table. One table may contain one or more INDEX tables.
An index helps to speed up SELECT queries and WHERE clauses, but it slows down data input, with the UPDATE and the INSERT statements as indexes need to be updated as well
Indexes can be created or dropped with no effect on the data
example:
CREATE INDEX index_name
on TABLE_NAME (Column1, Column2…);
Indexes are used to retrieve data from the database more quickly than otherwise. The users cannot see the indexes, they are just used to speed up searches/queries.
Note: Updating a table with indexes takes more time than updating a table without (because the indexes also need an update). So, only create indexes on columns that will be frequently searched against.
CREATE INDEX Syntax
Creates an index on a table. Duplicate values are allowed:
CREATE INDEX index_name
ON table_name (column1, column2, …);
CREATE UNIQUE INDEX Syntax
Creates a unique index on a table. Duplicate values are not allowed:
CREATE UNIQUE INDEX index_name
ON table_name (column1, column2, …);
Indexes are special lookup tables that the database search engine can use to speed up data retrieval. Simply put, an index is a pointer to data in a table. An index in a database is very similar to an index in the back of a book.
For example, if you want to reference all pages in a book that discusses a certain topic, you first refer to the index, which lists all the topics alphabetically and are then referred to one or more specific page numbers.
Creating an index involves the CREATE INDEX statement, which allows you to name the index, to specify the table and which column or columns to index, and to indicate whether the index is in an ascending or descending order.
CREATE INDEX index_name ON table_name;
Single Column:
CREATE INDEX index_name
ON table_name (column_name);
Multiple columns:
CREATE INDEX index_name
on table_name (column1, column2);
Indexes can also be unique, like the UNIQUE constraint, in that the index prevents duplicate entries in the column or combination of columns on which there is an index.
CREATE UNIQUE INDEX index_name
on table_name (column_name);
Indexing is a procedure that returns your requested data faster from the defined table. Without indexing, the SQL server has to scan the whole table for your data. By indexing, SQL server does the exact same thing when you search for a content in a book by checking the index page. In the same way table’s index allows us to locate the exact data without scanning the whole table.
There are two types of indexing in SQL –
1. Clustered index
2. Non-clustered index
Examples can be found here – https://www.geeksforgeeks.org/sql-queries-on-clustered-and-non-clustered-indexes/?ref=rp
https://www.geeksforgeeks.org/difference-between-clustered-and-non-clustered-index/?ref=rp
Indexes are used to retrieve data from the database more quickly than otherwise. The users cannot see these indexes, they are used to speed up the search/queries.
NOTE- Updating the table with indexes takes more time than updating a table without indexes because the indexes are also updated. Therefore, only create indexes for columns against which they will frequently searched.
Syntax:
CREATE INDEX index_name
ON table_name(col1, col2, …………);.