Redshift Spectrum:

• Amazon Redshift Spectrum is used to run queries against exabytes of unstructured data in Amazon S3, with no loading or ETL required.
• Redshift Spectrum queries employ massive parallelism to execute very fast against large datasets. Much of the processing occurs in the Redshift Spectrum layer, and most of the data remains in Amazon S3.
• Redshift Spectrum queries use much less of your cluster’s processing capacity than other queries.
• The cluster and the data files in Amazon S3 must be in the same AWS Region.
• External S3 tables are read-only. You can’t perform insert, update, or delete operations on external tables.