Mergeout request types and precedence

The Tuple Mover constantly monitors all activity that generates new ROS containers.

The Tuple Mover constantly monitors all activity that generates new ROS containers. As it does so, it creates mergeout requests and queues them according to type. These types include, in descending order of precedence:

RECOMPUTE_LIMITS: Sets criteria used by the Tuple Mover to determine when to queue new merge requests for a projection. This request type is queued in two cases:
- When a projection is created.
- When an existing projection changes—for example, a column is added or dropped, or a configuration parameter changes that affects ROS storage for that projection, such as ActivePartitionCount.
MERGEOUT: Consolidate new containers. These containers typically contain data from recent load activity or table partitioning.
DVMERGEOUT: Consolidate data marked for deletion, or delete vectors.
PURGE: Purge aged-out delete vectors from containers.

The Tuple Mover also monitors how frequently containers are created for each projection, to determine which projections might be at risk from ROS pushback. Intense DML activity on projections typically causes a high rate of container creation. The Tuple Mover monitors MERGEOUT and DVMERGEOUT requests and, within each set, prioritizes them according to their level of projection activity. Mergeout requests for projections with the highest rate of container creation get priority for immediate execution.

Note

The Tuple Mover often postpones mergeout for projections with a low level of load activity. Until a projection meets the internal threshold for queuing mergeout requests, mergeout from those projections is liable to remain on hold.