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 does not allow 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 transaction associated with the column-based in-memory database is received. For each table read by the transaction, a version of the table in the delta store that represents a transaction-consistent snapshot of the database visible to the transaction is determined. Each table is represented in the main store and the delta store; and each version of the table is represented by one or more bitmaps. Upon execution of a DML as part of the transaction, for each table written by the transaction, the data changes generated by the transaction is recorded in the one or more bitmaps that represent a private version of the table. Upon commit of the transaction, for each table written by the transaction, a new public version of the table is generated based on the private version of the table, and the public version represents a new transaction-consistent snapshot of the database visible to subsequent transactions.