1. A processor-implemented method for partitioning a database index, the method comprising: after determining that a first-level index partition stored in a data cache exceeds a threshold size, performing a first-level partitioning operation to i) write the first-level index partition stored in the data cache to a secondary storage, and ii) generate a new first-level index partition in the data cache; and after determining that a number of first-level index partitions in the secondary storage has reached a predetermined number, performing a first-level merge operation to i) merge the predetermined number of first-level index partitions into a single second-level index partition, and ii) delete the predetermined number of first-level index partitions from the secondary storage.