Lesson 1 of 0
In Progress

Understanding and Using Indexes

Kedeisha November 23, 2023

In SQL, an index is a special lookup table that the database search engine can use to speed up data retrieval. Simply put, an index in a database is akin to an index in a book. It consists of pointers to the rows in a table and is sorted by the column(s) that you choose. This sorting allows the database to find data quickly, particularly for large tables, much like an index in a book helps you to find specific information quickly.

How Indexes Improve Performance:

  • Without an index, the database must go through each row in the table to find relevant rows (full table scan). This process is slow for large tables.
  • With an index, the database can jump to the relevant part of the table directly, significantly reducing the search time.
  • Indexes are particularly effective for SELECT queries with WHERE clauses and JOIN operations.

Explanation of When and How to Create Indexes

When to Create Indexes:

  • Columns Used in WHERE Clauses: If you frequently retrieve rows from a table based on a column’s value, consider indexing that column.
  • Join Columns: Columns that are often used to join tables should usually be indexed.
  • High Cardinality Columns: Columns with a high number of unique values, like email addresses or IDs, are good candidates for indexing.
  • Limitations: Be cautious about over-indexing, as each index requires additional space and can slow down write operations like INSERT, UPDATE, and DELETE.

How to Create Indexes:

  • Basic Syntax:
CREATE INDEX index_name ON table_name (column1, column2, ...);

Managing Indexes

  • Viewing Indexes: Most SQL databases provide a way to view existing indexes on a table. The method varies depending on the database system.
  • Removing Indexes: When an index is no longer needed or is negatively impacting write performance, it can be removed.


Indexes are a powerful feature in SQL databases that can drastically improve the performance of query operations. However, they should be used judiciously, as they come with trade-offs, especially in terms of storage space and potential impacts on the performance of data write operations. Understanding when and how to use indexes effectively is key to optimizing the performance of a database.