What is an index in a database and why is it needed?

An index in a database is a special data structure that speeds up the search and sorting of records in a table. Essentially, an index is a pointer to data that allows quick access to the needed rows without scanning the entire table.

Indexes are necessary to improve the performance of SELECT queries, especially when the table is large and queries filter by certain columns. Without an index, the search will be slow because the DBMS has to scan all records.

For example: if you have a user table with millions of records and you often search for a user by email, creating an index on the email field will allow you to find the user very quickly.