What types of indexing in databases have you used, and for which scenarios were they most effective?

In my practice, I have used the following types of indexing:

• B-Tree indexes — the most versatile, suitable for fast searches, sorting, and range queries. Used for fields with unique values, such as ID or email.
• Hash indexes — effective for exact key searches but do not support range queries. Applied for quick lookups by unique keys in some NoSQL databases.
• Full-text indexes — for text searches, such as in descriptions or comments.

The choice of index depends on the scenario:

• For frequent point queries — hash or unique B-Tree indexes.
• For sorting and range queries — B-Tree.
• For text search — full-text indexes.

Proper indexing significantly speeds up queries, but excessive indexes slow down data insertion and updates.