SQL DROP INDEX Statement
In SQL, indexes play an essential role in optimizing query performance by speeding up data retrieval operations. However, indexes can also slow down data modification operations such as INSERT, UPDATE, and DELETE due to the overhead of maintaining them. When an index is no longer required or becomes redundant, it's important to remove it using the DROP INDEX statement
In this article, we will explain the purpose of the SQL DROP INDEX command, how to use it, and explore different examples and scenarios for removing indexes in SQL databases.
What is SQL DROP INDEX?
The SQL DROP INDEX Command is used to remove an existing index from a table. Indexes are created to speed up query performance, but they can also consume storage space and add overhead to write operations. By removing unnecessary indexes, we can free up space and improve the performance of INSERT, UPDATE, and DELETE operations.
Indexes created by PRIMARY KEY or UNIQUE constraint can not be deleted with just a DROP INDEX statement. To delete such indexes, we need to first drop the constraints using the ALTER TABLE statement, and then drop the index.
Syntax for DROP INDEX Statement
The syntax for dropping an index varies slightly depending on the database system we are using. Here are the syntaxes for different database management systems (DBMS):
MySQL
ALTER TABLE table_name DROP INDEX index_name;
MS Access
DROP INDEX index_name ON table_name;
SQL Server
DROP INDEX table_name.index_name;
DB2/Oracle
DROP INDEX index_name;
PostgreSQL
DROP INDEX index_name;
Example of SQL DROP INDEX
Let’s go through an example where we create a table and add an index using the CREATE INDEX statement, and then drop it using the DROP INDEX statement.
Step 1: Create the EMPLOYEE Table
CREATE TABLE EMPLOYEE(
EMP_ID INT,
EMP_NAME VARCHAR(20),
AGE INT,
DOB DATE,
SALARY DECIMAL(7,2));
Step 2: Create an Index on the EMPLOYEE Table
CREATE INDEX EMP
ON EMPLOYEE(EMP_ID, EMP_NAME);
This creates an index named EMP
on the EMP_ID
and EMP_NAME
columns of the EMPLOYEE table, improving query performance for queries involving these columns.
Dropping an Index
Now let's look at some examples of DROP INDEX statement and understand its workings in SQL. We will learn different use cases of the SQL DROP INDEX statement with examples. We can drop the index using two ways either with IF EXISTS or with ALTER TABLE so we will first drop the index using if exists.
Example 1: SQL DROP INDEX with IF EXISTS
Using the IF EXISTS clause ensures that the index is dropped only if it already exists in the table. This prevents errors from being thrown if the index is not present.
Query:
DROP INDEX IF EXISTS EMP ON EMPLOYEE;
Output
Commands Executed Successfully;
Explanation:
This query drops the EMP
index from the EMPLOYEE table only if the index exists, ensuring no error occurs if the index is absent. Since the EMP
index exists, it is successfully removed. If the index didn’t exist, no error would be thrown
Example 2: SQL DROP INDEX with ALTER TABLE
In MySQL, PostgreSQL, and some other databases, the ALTER TABLE statement is used to drop an index. This query drops the EMP
index on the EMPLOYEE table using the ALTER TABLE statement.
Query:
ALTER TABLE EMPLOYEE
DROP INDEX EMP;
Output

Verifying the DROP INDEX
To verify if the DROP INDEX statement has successfully removed the index from the table, we can check the indexes on the table. If the index is not present in the list, we know it has been deleted. This can be done using database-specific commands.
MySQL Verification:
SHOW INDEXES FROM EMPLOYEE;
SQL Server Verification:
SELECT * FROM sys.indexes WHERE object_id = (SELECT object_id FROM sys.objects WHERE name = 'EMPLOYEE');
PostgreSQL Verification:
SELECT * FROM USER_INDEXES WHERE table_name = 'EMPLOYEE';
Explanation:
These queries retrieve information about the indexes associated with the EMPLOYEE table. If the index was successfully removed, it will not appear in the results.
Important Points About SQL DROP INDEX Statement
- The SQL DROP INDEX statement is used to remove an existing index from a database table.
- It optimizes database performance by reducing index maintenance overhead.
- It improves the speed of improve INSERT, UPDATE, and DELETE operations on the table.
- Indexes created by PRIMARY KEY or UNIQUE constraints cannot be dropped using just the DROP INDEX statement.
- The IF EXISTS clause can be used to conditionally drop the index only if it exists.
- To verify if the DROP INDEX statement has successfully removed the index from the table, we can check the indexes on the table.
Conclusion
The SQL DROP INDEX statement is an important tool for database optimization. By removing unnecessary indexes, we can free up space, improve write performance, and enhance overall system efficiency. However, we should always evaluate the impact of dropping an index on query performance before proceeding, especially for frequently accessed columns. Indexes should be used strategically, and removing them should only be done when they are no longer required, ensuring a balanced and efficient database environment.