function initializeUnique() {
  uniquecoll = db.addCollection('uniquecoll', {
    unique: ['customId']
  });

  for (var idx = 0; idx < arraySize; idx++) {
    var v1 = genRandomVal();
    var v2 = genRandomVal();

    uniquecoll.insert({
      customId: (arraySize - idx),
      val: v1,
      val2: v2,
      val3: "more data 1234567890"
    });
  }
}
initializeUnique : to support benchUniquePerf, we will set up another collection
where our customId is enforced as 'unique' using unique index feature of loki.