A multi-level storage architecture and method of performing a partial merge are disclosed. A main store is partitioned into a passive main part and an active main part, the active main part being empty at a start of the partial merge, the passive main part storing encoded data records of the main store that are not subject to the partial merge. A values index corresponding to a sorted dictionary of the passive main part is set to a cardinality of n. The data records of a second level storage structure is merged into the active main part, the active main part having a dictionary that starts with a value of n+1, such that the merging into the active main part continues an encoding scheme according to the values index of the passive main part.