A delta store giving row-level versioning semantics to a non-row-level versioning underlying store is described. An example method includes establishing a column-based in-memory database including a main store and a delta store, where the main store allows only non-concurrent transactions on a same table and the delta store has a plurality of row-visibility bitmaps implementing a row-level versioning mechanism that allows concurrent transactions on the same table. A local RID space is established for a table fragment, that for each table in the database, the data of the table is stored in one or more main table fragment in the main store and in one or more delta table fragments in the delta store. Each table fragment has a local RID space, and the local RID space is a collection of one-based contiguous integer local RIDs (Row IDs) describing local positions of the rows of the table fragment.