ment of such an algorithm is that it minimize the number of random disk seeks during sorting – sequential disk reads are far faster than seeks as we explained in Section 4.1. One solution is the blocked sort-based indexing algorithmor BSBI in Figure 4.2. BSBI(i)segments the collection into parts of equal size, (ii) sorts the termID–docID pairs of each part in memory, (iii) stores in- termediate sorted results on disk, and (iv) merges all intermediate results into the final index.