arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a" ], sparse: true });
{ 
  "id" : "test/1130951563", 
  "type" : "hash", 
  "fields" : [ 
    "a" 
  ], 
  "selectivityEstimate" : 1, 
  "unique" : false, 
  "sparse" : true, 
  "isNewlyCreated" : true, 
  "code" : 201 
}
arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a", "b" ], unique: true });
{ 
  "id" : "test/1131213707", 
  "type" : "hash", 
  "fields" : [ 
    "a", 
    "b" 
  ], 
  "selectivityEstimate" : 1, 
  "unique" : true, 
  "sparse" : false, 
  "isNewlyCreated" : true, 
  "code" : 201 
}