The current subject matter describes static partitioning and sub-partitioning of a row identifier space associated with a table in a delta memory store of a database so as to allow data to be concurrently inserted into rows identified by the corresponding sub-partitions. A server system associated with the database can receive data to be inserted into a database. The server system can select a sub-fragment of a row identifier space identifying identifiers of rows stored in the database for the table. The sub-fragment can be selected based on a preference specified by an insert operation used for insertion of the data into the columnar database and on availability of the sub-fragment. The server system can insert the data in rows identified by the selected sub-fragment while other data is being concurrently inserted in rows identified by other one or more sub-fragments of the row identifier space.