The authors have developed an information retrieval system named AIR (Augmented Information Retrieval system), which might be one of the most efficient systems for very large document databases. AIR can store the document data compactly and retrieve them quickly. The techniques bringing AIR to the high efficiency, the data compression, the quick keyword index, and the automatic keyword selection, are discussed. These techniques, which are based on the statistical properties of word occurrence, are fairly simple, so that the information retrieval systems employing them can be implemented with ease. The data compression technique reduces English text by a factor of 4. The quick keyword index decreases the average number of disk accesses to retrieve a keyword to about 0.3. The automatic keyword selection technique roughly halves both the number of different keywords and the size of the inverted file with only 2% loss of retrieval power.