Real World Peformance of SQL vs Core Data: Need Perspective and Perhaps Specific Fixes - objective-c

Question

I have a word game app that accesses a list of words and clues, which is about 100K entries. The data base is only accessed, never changed. I built the app using SQL methods and it performs pretty well on iOS 6 but the time to get a new clue from the data base is extremely slow on iOS 5:
iOS 5, getting one record from the 100K using SQL, takes about 12
seconds.
iOS 6, getting one record from the 100K using SQL, takes
about 700-1000 milliseconds.
Both of these are on 32 GB iPod Touch.
Given this performance, I made a version using Core Data. My approach gets a random data base entry by first counting the records that fit the query, then chosing one at random. Code follows. Everything I read suggested that Core Data would be faster:
iOS 5, counting records takes around 4 seconds, and retrieving one of
those records takes about 50 - 1500 millisecs. A total time of about 5 seconds.
iOS 6, counting records takes a bit over 2 seconds, and retrieving
one of those records takes about 300-500 millisecs. A total of about 3 seconds.
So Core Data is faster on iOS 5 but slower on iOS 6, compared to SQL. Either way the performance is too slow as far as I am concerned. I know the overhead comes from the methods given below (for the Core Data version). So, two questions:
Any general advice about this issue with an eye to understanding it and improving performance?
Specifically, what about the Core Data code appended below: have I done something foolish that slows it down? Or something else I should add to speed it up? This is my first attempt at Core Data.
Thanks.
- (NSArray *) randomClue {
NSManagedObjectContext* context = [self managedObjectContext];
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:#"A"];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription
entityForName:#"WL28"
inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSPredicate *predicate = [self createSearchQuery];
[fetchRequest setPredicate:predicate];
NSError *error;
NSDate *date1 = [NSDate date];
NSString *timeString1 = [formatter stringFromDate:date1];
int resCount = [context countForFetchRequest:fetchRequest
error:&error];
NSDate *date2 = [NSDate date];
NSString *timeString2 = [formatter stringFromDate:date2];
int t1 = [timeString1 intValue];
int t2 = [timeString2 intValue];
int d1 = t2-t1;
NSLog(#"randomClue:");
NSLog(#" Time to count array entries: %i", d1);
int ranNum = arc4random_uniform(resCount-1);
int ranNum2 = ranNum + 1;
// Now we fetch just one answer object, not a whole database or even a piece of it!
[fetchRequest setReturnsObjectsAsFaults:YES];
[fetchRequest setPropertiesToFetch:nil];
[fetchRequest setFetchLimit:1];
[fetchRequest setFetchOffset:ranNum2];
NSDate *date3 = [NSDate date];
NSString *timeString3 = [formatter stringFromDate:date3];
self.wl28 = [context executeFetchRequest:fetchRequest error:&error];
NSDate *date4 = [NSDate date];
NSString *timeString4 = [formatter stringFromDate:date4];
int t3 = [timeString3 intValue];
int t4 = [timeString4 intValue];
int d2 = t4-t3;
NSLog(#" Time to retrieve one entry: %i", d2);
return self.wl28;
}
EDIT: createSearchQuery added below
- (NSPredicate *)createSearchQuery {
NSMutableArray *pD = [[GameData gameData].curData valueForKey:#"persData"];
NSNumber *currMin = [pD objectAtIndex:0];
NSNumber *currMax = [pD objectAtIndex:1];
NSNumber *dicNo = [pD objectAtIndex:2];
NSString *dict = nil;
if ([dicNo intValue] == 0) dict = #"TWL";
if ([dicNo intValue] == 1) dict = #"LWL";
NSPredicate *dictPred = [NSPredicate predicateWithFormat:#"dict == %#", dict];
NSPredicate *lowNoCPred = [NSPredicate predicateWithFormat:#"noC >= %#", currMin];
NSPredicate *highNoCPred = [NSPredicate predicateWithFormat:#"noC <= %#", currMax];
NSPredicate *query = [NSCompoundPredicate andPredicateWithSubpredicates:[NSArray
arrayWithObjects:dictPred, lowNoCPred, highNoCPred,nil]];
NSLog(#"%#", query);
return query;
}

Answer 1

You can try adding index to your noC and/or dict attributes inside your entity. It might speed up your query time.