How do you create and manage indexes in an Oracle database? — Oracle Database Interview Question #26

Indexes play a significant role in improving the performance of database queries. When a query is executed, the database engine scans through the data to find the required results. With large amounts of data or when queries require multiple joins or filtering conditions, queries can become very resource intensive and take longer to complete. An index provides a structure for storing and retrieving data more efficiently, speeding up queries.

Creating an Index in Oracle Database

To create an index in Oracle Database, you can use the ‘CREATE INDEX‘ statement. The basic syntax is as follows:

CREATE INDEX index_name
ON table_name (column1, column2, ...);

For example, let’s say you have a table ‘employees‘ with the following columns:

CREATE TABLE employees(
   emp_id NUMBER PRIMARY KEY,
   first_name VARCHAR2(100),
   last_name VARCHAR2(100),
   email VARCHAR2(100) UNIQUE,
   hire_date DATE
);

You can create an index on the ‘last_name‘ column using the following command:

CREATE INDEX employees_last_name_ix
ON employees(last_name);

The above statement creates an index named ‘employees_last_name_ix‘ on the ‘last_name‘ column of the ‘employees‘ table.

Managing Indexes in Oracle Database

Once the index is created, you can manage it using several commands.

1. Viewing Index Information: You can view information about an index using the ‘DBA_INDEXES‘ or ‘USER_INDEXES‘ views depending on the level of access you have.

SELECT index_name, table_name, column_name
FROM USER_IND_COLUMNS
WHERE table_name = 'employees';

2. Altering an Index: You can alter an index to change its structure or options using the ‘ALTER INDEX‘ statement. For example, you may need to add a new column to the index so it covers more columns needed for the query.

ALTER INDEX employees_last_name_ix
ADD emp_id;

3. Rebuilding an Index: In some cases, the index may become invalid or inefficient. In these cases, the index requires rebuilding using the ‘ALTER INDEX...REBUILD‘ statement. For example, you may rebuild the index on a daily basis or whenever you add new rows to the table, especially when clustered.

ALTER INDEX employees_last_name_ix
REBUILD;

4. Dropping an Index: In case you no longer require an index, you can drop it from the database using the ‘DROP INDEX‘ statement.

DROP INDEX employees_last_name_ix;

Conclusion

Creating and managing indexes in Oracle Database plays a significant role in improving query performance. Therefore, it’s essential to understand how to create, view, alter, rebuild, and drop indexes based on the database performance concerns.