What are indexes in SQL and what are they used for? What are the disadvantages?

Indexes in SQL are special data structures that speed up the search and sorting of data in tables. Essentially, an index is an additional table that stores the values of one or more columns and links to the corresponding rows of the main table.

Indexes are needed for:

• Fast searching of records by key fields (e.g., by ID or unique name).
• Accelerating JOIN operations and ORDER BY sorting.

Disadvantages of indexes:

• They occupy additional disk space.
• They slow down insert, update, and delete operations because the index also needs to be updated.
• Excessive indexes can degrade overall performance.

Example of creating an index:

CREATE INDEX idx_user_email ON users(email);

This index will speed up searching for users by email.