What data structure is usually at the core of the indexing system in a database?

The core of the indexing system in a database is usually based on a data structure called a B-tree (or its variations, such as B+-tree).

A B-tree is a balanced search tree that efficiently supports insert, delete, and search operations in logarithmic time. It is optimized for working with large amounts of data on disk, minimizing disk access.

B-tree-based indexes allow quick record retrieval by key, significantly speeding up query execution in databases.