Pick always a different number in a range of 3K numbers

Question

i'm builing an hybrid app in Javascript that casually pick an exercise from a database, wait for the user to solve it and then pick another exercise.

Nothing difficult at this point, my problem is about picking always a different exercise from database.

Now my code 'works' in this way:

1. Gets total number of exercises (about 3K),
2. Generate a N random number from 1 to 3000,
3. Pick the N exercise from DB,
4. Save the N number in the 'used' db table,
5. Generate another number name,
6. This time first of all check if the generated number was already used,
7. If NO pick exercise, if YES generate another number,
8. etc...

The code works well for the first 2000-2500 numbers, then start to be really slow because generating a random number not already used starts to be very difficult. When the number used are just a bit less than 3K the code is totally broken: continues to check but the new number never comes

How can i handle this situation in a fast and light way that use memory as less as possible?

Thank you very much!

EDIT

I'm using WebSQL in a phonegap application

That's what i'm doing (code):

Here i get the number of the exercises

1. SELECT COUNT(*) AS countArt FROM Exercises'

Here i pick a random number from 1 to totalNumberOfExercises

2. randomNumber = random(totalNumberOfExercises);
   function numeroCasuale(len){
        var num = Math.floor(Math.random() * len) + 0;
        return num;
    };

Here i check if my randomNumber is already in the DB, that means is already used

4. SELECT EXISTS(SELECT * FROM alreadyUsed WHERE numero=randomNumber)

If randomNumber exists i generate another randomNumber and check it again and again...

Answer 1

Using this shuffle function from another stackoverflow question:

function shuffle(o){
    for(var j, x, i = o.length; i; j = Math.floor(Math.random() * i), x = o[--i], o[i] = o[j], o[j] = x);
    return o;
}

You can create this:

function generateRandomSequence(amount){
    var random = [];
    for (var i = 0; i<amount; i++) random[i] = i;
    return shuffle(random);
}

var random = generateRandomSequence(3000);
var gamesCompleted = 0;

function nextGame(){
    console.log(random[gamesCompleted]);
    gamesCompleted++;
}

nextGame();

This creates an array where position 0 = 0, 1 = 1 and so on and shuffle it so we have a random sequence without doublicates. Keep track of how many games you have completed and use this number as the index for the next question. Here's a fiddle.