title: How are inverted index stored?
body: <p>I recently made an index of approx. 2,000,000 documents in memory. The documents are imported from mysql datbase and takes about 6 to 10 seconds to load. Every time when I start the program time is consumed in importing data. I have tried using json, pickle, cPickle and even redis but time is concern and for update I have to restart the whole program. I am using python here.</p>
  
  <p>My question is that how search engines like google, solr, elasticsearch stores inverted indexes. Do they store them in memory as hash-tables or in database? How index are updated without restart? What can be the best database for such purpose.</p>