In which cases does it make sense to use an index?

An index in databases makes sense to use when you need to speed up search, filtering, or sorting operations on certain fields. This is especially relevant when working with large volumes of data, where a full table scan is too costly in terms of time.

Indexes are useful if:

• Queries with WHERE conditions on the indexed field are frequently executed.
• JOINs are performed on indexed columns.
• You need to quickly retrieve sorted data.

However, indexes occupy additional space and slow down insert, update, and delete operations, as the index also needs to be updated. Therefore, you should not index everything, only those fields that are actually involved in search and filtering.

Example in Go using SQL:

// Suppose we have a table users with an index on email
rows, err := db.Query("SELECT id, name FROM users WHERE email = ?", email)
// The query will be fast thanks to the index on email

In summary: it makes sense to use an index to speed up queries on fields that are frequently used in conditions, especially with large data volumes.