Explain what partial indexes in databases are and in which cases they are used.

Partial indexes are indexes that are created not on the entire table, but only on a subset of rows that satisfy a certain condition (filter). This allows reducing the size of the index and speeding up search operations if queries often access a subset of data.

For example, if a table of orders frequently queries only active orders, a partial index can be created on rows with status = 'active'. Then, the index will contain only these records, saving space and increasing performance.

Example in SQL:

CREATE INDEX idx_active_orders ON orders(order_date) WHERE status = 'active';

Partial indexes are useful when data has a clear division by status or categories, and queries are focused on specific subsets.