Index Compression Why compression for inverted indexes? Dictionary Make it small enough to keep in main memory Make it so small that you can keep some postings lists in main memory too Posting gs file(s) Reduce disk space needed Decrease time needed to read postings lists from disk Large search engines keep a significant part of the postings in memory. Compression lets you keep more in memory We will devise various IR-specific compression schemesIndex Compression 5 Why compression for inverted indexes? ▪ Dictionary ▪ Make it small enough to keep in main memory ▪ Make it so small that you can keep some postings lists in main memory too ▪ Postings file(s) ▪ Reduce disk space needed ▪ Decrease time needed to read postings lists from disk ▪ Large search engines keep a significant part of the postings in memory. ▪ Compression lets you keep more in memory ▪ We will devise various IR-specific compression schemes