Mergeout is a Tuple Mover process that consolidates ROS containers and purges deleted records. DML activities such as COPY and data partitioning generate new ROS containers that typically require consolidation, while deleting and repartitioning data requires reorganization of existing containers. The Tuple Mover constantly monitors these activities, and executes mergeout as needed to consolidate and reorganize containers. By doing so, the Tuple Mover seeks to avoid two problems:
- Performance degradation when column data is fragmented across multiple ROS containers.
- Risk of ROS pushback when ROS containers for a given projection increase faster than the Tuple Mover can handle them. A projection can have up to 1024 ROS containers; when it reaches that limit, Vertica starts to return ROS pushback errors on all attempts to query the projection.