Novel methods are described herein to provide an OLAP database system that performs lock-less bulk insertion while maintaining high query performance and minimizing additional storage requirements. Aspects of the claimed subject matter include novel methods for merging two index structures while maintaining query consistency without the use of a versioning scheme by using copies of vertical segments of an index tree page; a method to prepare and merge two index structures that operates within storage constraints; and a method to execute queries concurrently while index maintenance is in progress while still producing consistent results. According to an aspect of the present disclosure, a flexible index merge mechanism is provided that merges data from incremental indices to a primary data index in three phases: a load phase, a copy phase, and a replace phase.