Why do indexes incur a write performance cost?

Indexes incur a write performance cost primarily because the index structure must be updated whenever a document is added, modified, or removed from the database. When a write operation occurs, the database must not only update the document itself but also ensure that the corresponding index entries reflect this change. This additional overhead can slow down write performance, particularly in systems with many indexes or frequent write operations.

Each index serves as a separate data structure that optimizes read operations by allowing quick lookups, but maintaining that structure requires resources and time. Therefore, every write to the underlying data necessitates a corresponding update to any indexes that depend on that data. This is why options such as those suggesting manual management or that indexes affect only read operations do not accurately reflect the true nature of the performance costs associated with writing operations. The notion of data duplication can also mislead; while indexes do involve additional storage, the performance implications are chiefly about the maintenance of index structures during writes.