What indexes have you had to use (including covering indexes)?

In Java development practices with databases, various types of indexes are often used to optimize queries:

• B-tree indexes — standard indexes for fast search by column values.
• Covering indexes — contain all fields needed to execute a query, allowing data to be retrieved directly from the index without accessing the table.
• Unique indexes — ensure the uniqueness of values in a column or set of columns.
• Composite key indexes — index multiple columns, useful for queries filtering by several fields.
• Function or expression-based indexes — allow indexing the result of a function on a column.

Example of a covering index for a query:

CREATE INDEX idx_user_email_status ON users (email, status);

If the query only selects email and status, this index will enable the query to be executed without accessing the main table, speeding up execution.

In Java applications, indexes are created at the database level, and ORM tools (like Hibernate) can assist in defining them through annotations or migrations.