Fetches a data set in pieces using limits and offsets.

LimitAndOffset.m

#define kBatchSize 20

// This could take a while.
+ (void) recalculateAllStats {
    MyAppDelegate *myAppDelegate = (MyAppDelegate *) 
        [[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *moc = 
        [myAppDelegate managedObjectContext];
    NSEntityDescription *entity = 
        [NSEntityDescription entityForName:@"Entry"
            inManagedObjectContext:moc];
    
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

    [fetchRequest setEntity:entity];
    // I need to look at all objects, so do not return faults.
    [fetchRequest setReturnsObjectsAsFaults:NO];
    [fetchRequest setFetchLimit:kBatchSize];
    [fetchRequest setFetchOffset:0];
    
    // Sort, in case Core Data doesn't enforce select order.
    NSSortDescriptor *sorting = 
        [[NSSortDescriptor alloc] 
            initWithKey:@"Date" ascending:NO];
    [fetchRequest setSortDescriptors:
        [NSArray arrayWithObject:sorting]];
    
    // Execute
    NSError *error;
    NSArray *items = 
        [moc executeFetchRequest:fetchRequest error:&error];

    int i = 0;
    int total = 0;
    // Continue executing until 0 results are returned.
    while ([items count] > 0) {

        // Use the data here.

        total += [items count];
        // Update offset.
        i = i + 1;
        [fetchRequest setFetchOffset:kBatchSize * i];
        items = 
            [moc executeFetchRequest:fetchRequest error:&error];
    }

    NSLog(@"Total = %d", total);
    
    [sorting release];
    [fetchRequest release];
}