Nutch 2.3.1 REST calls to DB

[Vladimir Loubenski]

Hi,
Nutch 2.x REST API documentation is mentioned  following syntax for DB calls  
:https://wiki.apache.org/nutch/NutchRESTAPI

1. What does mean  "startKey", "endKey" and  "isKeysReversed" ?
POST /db
   {
      "startKey":"com.google",
      "endKey":"com.yahoo",
      "isKeysReversed":"true"
   }

Call bellow doesn't work for me. It always return empty result   
POST /db
   {
      "batchId": "batch-id"
   }


Thank you in advanse.
Vladimir.