Various approaches for adding data items to a database are described. In one approach, a method includes receiving a plurality of data items; each data item is to be stored under a unique primary key in the database. In response to each received data item, one of a plurality of fragment builders is selected and the data item is provided as input to the selected fragment builder. The fragment builders operate in parallel to create respective pluralities of B-tree fragments from the input data items. The B-tree fragments are merged into a single B-tree of the database, which is then stored.