What is an index?

Question

What is an index in MySQL ????

Answer 1

Indexes speed up SELECT queries because indexes are sorted by definition. Additionally a UNIQUE index enforces the constraint that the value of that column (or the combination of values of bound columns) exists only once (same applies to PRIMARY, but PRIMARY can only exist once per table in contrast to a UNIQUE key).

Indexes are a tradeoff: they tremendously speed up SELECT queries (when used columns have an INDEX) but they make that MySQL table consume more space and cost more time when changing the table via INSERT/UPDATE/DELETE.