title: How MongoDB manage secondary indexes scans?
body: <p>By default MongoDB creates index on _id key in document. But when I ensure additional index (secondary like in InnoDB from MySQL?) and query it after, engine scans it and then selective scan _id index to get documments offsets?</p>
  
  <p>I'm confused because when sharding comes it I'm right every chunk have own indexes and there will be many random reads per query?</p>