druid/docs/data-management/schema-changes.md

id	title
schema-changes	Schema changes

For new data

Apache Druid allows you to provide a new schema for new data without the need to update the schema of any existing data. It is sufficient to update your supervisor spec, if using streaming ingestion, or to provide the new schema the next time you do a batch ingestion. This is made possible by the fact that each segment, at the time it is created, stores a copy of its own schema. Druid reconciles all of these individual segment schemas automatically at query time.

For existing data

Schema changes are sometimes necessary for existing data. For example, you may want to change the type of a column in previously-ingested data, or drop a column entirely. Druid handles this using reindexing, the same method it uses to handle updates of existing data. Reindexing involves rewriting all affected segments and can be a time-consuming operation.