Have you had experience using partial indexes in a database?

Partial indexes are indexes that are created not on the entire table, but only on a subset of rows that satisfy a certain condition.

For example, in PostgreSQL, you can create an index only for active users:

CREATE INDEX idx_active_users ON users (email) WHERE active = true;

Such an index reduces size and improves the performance of queries that filter by the condition (e.g., WHERE active = true).

Experience with partial indexes is useful when most of the data is not involved in frequent queries, and you need to optimize specifically for the frequently used subsets.