How do you determine the need for indexing in SQL queries?

Indexing in SQL is used to speed up data search and retrieval. The need for its application is determined by the following signs:

• Frequent queries with WHERE, JOIN, or ORDER BY conditions on certain columns.
• Tables with a large number of rows, where full table scans take a lot of time.
• Query execution plan analysis (EXPLAIN) shows that queries are slow due to the absence of indexes.

Example: if you have a Users table with millions of records, and queries on Email are frequent, you should create an index:

CREATE INDEX idx_users_email ON Users(Email);

However, indexes slow down insert and update operations, so they should be used judiciously.