A method, system and program storage device are provided for extending an inverted index, which comprises first and second inverted index subfiles to increase the speed of establishing and updating inverted index files. The method includes performing ordered keyword indexing operations of generating an inverted index from data sources, in which a frequency of occurrence of keywords in each of the data sources is calculated, and writing each keyword, the data sources, and the frequency of occurrence of each keyword in the corresponding data sources to the inverted index. If a number of data sources involved in the indexing operations reaches a first threshold, then writing contents of the inverted index as a smallest grid into the first inverted index subfile. If a number of smallest grids in the first inverted index subfile reaches a second threshold, then merging the smallest grids into a merged grid and writing the merged grid into the second inverted index subfile. If the number of merged grids in the second inverted index subfile reaches a third threshold, then further merging the merged grids into a larger merged grid, and writing the larger merged grid back into the first inverted index subfile.