How are indexes implemented and how do they function in database management systems?

Indexes in DBMS are special data structures that accelerate the search and sorting of records in tables. Usually, an index is built on one or more columns and stored separately from the main table.

Most commonly, B-trees (B-tree) or their variations are used, which provide logarithmic search time. When creating an index, the DBMS forms a tree where the keys are the values of the indexed columns, and the leaves contain references to the corresponding records in the table.

When a query with a condition on an indexed column is executed, the DBMS accesses the index, quickly finds the necessary keys, and retrieves references to the records, bypassing a full table scan.

Indexes can be:

• unique — prohibit duplicate values;
• composite — index multiple columns;
• clustered — determine the physical order of data storage.

However, indexes occupy additional space and slow down insert and update operations, as they require maintaining the index structure in an up-to-date state.