# Mergeout strata algorithm

The mergeout operation uses a strata-based algorithm to verify that each tuple is subjected to a mergeout operation a small, constant number of times, despite the process used to load the data. The mergeout operation uses this algorithm to choose which ROS containers to merge for non-partitioned tables and for active partitions in partitioned tables.

Vertica builds strata for each active partition and for projections anchored to non-partitioned tables. The number of strata, the size of each stratum, and the maximum number of ROS containers in a stratum is computed based on disk size, memory, and the number of columns in a projection.

Merging small ROS containers before merging larger ones provides the maximum benefit during the mergeout process. The algorithm begins at stratum 0 and moves upward. It checks to see if the number of ROS containers in a stratum has reached a value equal to or greater than the maximum ROS containers allowed per stratum. The default value is 32. If the algorithm finds that a stratum is full, it marks the projections and the stratum as eligible for mergeout.