An Ω(1/ε log 1/ε) space lower bound for finding ε-approximate quantiles in a data stream

  • Authors:
  • Regant Y. S. Hung;Hingfung F. Ting

  • Affiliations:
  • The University of Hong Kong, Pofulam, Hong Kong;The University of Hong Kong, Pofulam, Hong Kong

  • Venue:
  • FAW'10 Proceedings of the 4th international conference on Frontiers in algorithmics
  • Year:
  • 2010

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Abstract

This paper studies the space complexity of the ε-approximate quantiles problem, which asks for some data structure that enables us to determine, after reading a whole data stream, a φ-quantile (for any 0 ≤ φ ≤ 1) of the stream within some error bound ε. The best known algorithm for the problem uses O(1/ε log εN) words where N is the total number of items in the stream, or uses O(1/ε log |U|) words where U is the set of possible items. It is known that the space lower bound of the problem is Ω(1/ε) words; however, improvement of this bound is elusive. In this paper, we prove that any comparison-based algorithm for finding ε-approximate quantiles needs Ω(1/ε log 1/ε) words.