Completer and Future in dart?

Question

Future readData() {
    var completer = new Completer();
    print("querying");
    pool.query('select p.id, p.name, p.age, t.name, t.species '
        'from people p '
        'left join pets t on t.owner_id = p.id').then((result) {
      print("got results");
      for (var row in result) {
        if (row[3] == null) {
          print("ID: ${row[0]}, Name: ${row[1]}, Age: ${row[2]}, No Pets");
        } else {
          print("ID: ${row[0]}, Name: ${row[1]}, Age: ${row[2]}, Pet Name: ${row[3]},     Pet Species ${row[4]}");
        }
      }
      completer.complete(null);
    });
    return completer.future;
  }

The above is an example code taken from github SQLJocky Connector

I would like someone to explain me if possible why is the function which has a completer object created outside the pool.query is then calling a function completer.complete(null).

In short I am not able to understand the part after print executes.

Note:Kindly if possible I would also like to know how is future and Completer used for practical purpose both for DB and non DB operations.

I have explored the following links: Google groups discussion on Future and Completer

and the api reference documentation which is as given below Completer api reference and Future api Reference

Answer 1

Correct answer has errors in DartPad, the reason could be Dart version.

error : The argument type 'int' can't be assigned to the parameter type 'Duration'.
error : The argument type '(dynamic) → void' can't be assigned to the parameter type '() → void'.

The following snippet is complement

import 'dart:async';

Future<dynamic> someFutureResult(){
   final c = new Completer();
   // complete will be called in 3 seconds by the timer.
   new Timer(Duration(seconds: 3), () {     
       print("Yeah, this line is printed after 3 seconds");
       c.complete("you should see me final");       
   });
   return c.future;

}

main(){
   someFutureResult().then((dynamic result) => print('$result'));
   print("you should see me first");
}

reslut

you should see me first
Yeah, this line is printed after 3 seconds
you should see me final