CoreData Sum Performance

I have some theoretical question to ask about Core Data and sum function. I try to sum values from Core Data table with three ways. fetch all and use expression to sum it up : ``` NSArray * array1 = [self getAll:self.managedObjectContext]; int sum = [[array1 valueForKeyPath:@"@sum.sum"] intValue]; ``` fetch all and use for loop: ``` int sum2 = 0; NSArray * array2 = [self getAll:self.managedObjectContext]; for (Test * t in array2) { sum2 = sum2 + [t.sum intValue]; } ``` let Core Data sum it. ``` NSArray * array = [self getAllGroupe:self.managedObjectContext]; NSDictionary * i = [array objectAtIndex:0]; id j = [i objectForKey:@"sum"]; (NSArray *)getAllGroupe:(NSManagedObjectContext*)Context{ NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Test" inManagedObjectContext:Context]; NSExpressionDescription* ex = [[NSExpressionDescription alloc] init]; [ex setExpression:[NSExpression expressionWithFormat:@"@sum.sum"]]; [ex setExpressionResultType:NSDecimalAttributeType]; [ex setName:@"sum"]; [fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:ex, nil]]; [fetchRequest setResultType:NSDictionaryResultType ]; NSError *error; [fetchRequest setEntity:entity]; NSArray *fetchedObjects = [Context executeFetchRequest:fetchRequest error:&error]; return fetchedObjects; } ``` surprisingly the - way was the slowest (`for 1.000.000 data --> 19.2 s`), the - way was faster (`for 1.000.000 data --> 3.54 s`) and the - way was the fastest (`for 1.000.000 data --> 0.3 s`) Why is this? If I understand right even core data need to go through all `1.000.000` datas and sum it. Is this because use more cores if there are available?