Sequence sorting in secondary storage

We investigate the I/O complexity of the problem of sorting sequences (or strings of characters) in external memory, which is a fundamental component of many large-scale text applications. In the standard unit-cost RAM comparison model, the complexity of sorting K strings of total length N is /spl Theta/(K log/sub 2/ K+N). By analogy, in the external memory (or I/O) model, where the internal memory has size M and the block transfer size is B, it would be natural to guess that the I/O complexity of sorting sequences is /spl Theta/((K/B)log/sub M/B/(K/B)+(N/B)), but the known algorithms do not come even close to achieving this bound. Our results show, somewhat counterintuitively, that the I/O complexity of string sorting depends upon the length of the strings relative to the block size. We first consider a simple comparison I/O model, where the strings are not allowed to be broken into their individual characters, and we show that the I/O complexity of string sorting in this model is /spl Theta/((N/sub 1//B)log/sub M/B/(N/sub 1//B)+K/sub 2/+(N/B)), where N/sub 1/ is the total length of all strings shorter than B and K/sub 2/ is the number of strings longer than B. We then consider two more general I/O comparison models in which string breaking is allowed. We obtain improved algorithms and in several cases lower bounds that match their I/O bounds. Finally, we develop more practical algorithms outside the comparison model.