Getting more from out-of-core columnsort

We describe two improvements to a previous implementation of out-of-core columnsort, in which data reside on multiple disks. The first improvement replaces asynchronous I/O and communication calls by synchronous calls within a threaded framework. Experimental runs show that this improvement reduces the running time to approximately half of the running time of the previous implementation. The second improvement uses algorithmic and engineering techniques to reduce the number of passes over the data from four to three. Experimental evidence shows that this improvement yields modest performance gains. We expect that the performance gain of this second improvement increases when the relative speed of processing and communication increases with respect to disk I/O speeds. Thus, as processing and communication become faster relative to I/O, this second improvement may yield better results than it currently does.