How do external keys affect the structure and performance of indexes in a database?

Foreign keys in a database ensure referential integrity between tables. When used, the DBMS typically automatically creates or requires indexes on the columns involved in the foreign key to efficiently enforce constraints during insert, update, and delete operations.

Impact on index structure:

• An index is often created on the foreign key column if it doesn't exist, to speed up lookups.
• Indexes help quickly find rows referenced by the foreign key, which is important for integrity checks.

Impact on performance:

• Positive: indexes speed up foreign key checks and join operations.
• Negative: data modifications (INSERT, UPDATE, DELETE) on tables with foreign keys and indexes impose additional overhead to maintain these indexes.

Thus, foreign keys influence index structure by necessitating additional indexes for integrity and performance, but this can increase overhead during data changes.