With the rapid increasing capacity of flash memory,flash-aware indexing techniques are highly desirable for flash devices.The unique features of flash memory,such as the erase-before-write constraint and the asymmetric read/write cost,severely deteriorate the performance of the traditional B+-tree algorithm.In this paper,we propose an optimized indexing method,called lazy-update B+-tree,to overcome the limitations of flash memory.The basic idea is to defer the committing of update requests to the B+-tree by buffering them in a segment of main memory.They are later committed in groups so that the cost of each write operation can be amortized by a bunch of update requests.We identify a victim selection problem for the lazy-update B+-tree and develop two heuristic-based commit policies to address this problem.Simulation results show that the proposed lazy-update method,along with a well-designed commit policy,greatly improves the update performance of the traditional B+-tree while preserving the query efficiency.