sort data using Realm - ios

Question

I have recently moved to Realm from Coredata. In my app I am showing 50K + contacts .
The contact object is in the format:
Contact: firstName, lastName ,company
I am trying to fetch all the contacts in the Realm , and I am trying to display those contacts similar to the native contacts app in iPhone.
First I am creating the section header titles based on the contact first name:
-(NSArray *)getSectionTitleBasedOn:(NSString*)sortBy{
RLMResults *results = [self getMainDataSetFromRealm];
ContactSource *contactSource = results.firstObject;
NSMutableDictionary *nameDic = [NSMutableDictionary dictionary];
for (RealmContact *contact in contactSource.contacts){
if (contact.firstName.length>0) {
if ([sortBy isEqualToString:#"FirstName"]) {
[nameDic setObject:#"firstletter" forKey:[contact.firstName substringToIndex:1]];
}
}
}
NSLog(#"dic %#",nameDic);
return [[nameDic allKeys]sortedArrayUsingSelector:#selector(localizedCaseInsensitiveCompare:)];
}
This gets me an array of letters which represent the title of section.
Now I am preparing the datasource for each section, so for section A, I am fetching all the contacts that begin with letter 'A'
-(void)prepareDataSource:(NSArray *)titleArr{
RLMResults *results = [self getMainDataSetFromRealm];
ContactSource *contactSource = results.firstObject;
__block NSMutableDictionary *dataSource = [NSMutableDictionary dictionary];
[titleArr enumerateObjectsUsingBlock:^(id _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
NSString *sectionHeader = obj;
RLMResults *contactResults = [contactSource.contacts objectsWhere:[NSString stringWithFormat:#"firstName BEGINSWITH '%#'",sectionHeader]];
NSMutableArray *contactRowArr = [NSMutableArray array];
for (Contact *contact in contactResults){
[contactRowArr addObject:contact];
}
[dataSource setObject:contactRowArr forKey:sectionHeader];
}];
_dataSource = [dataSource copy];
[self.tableView reloadData];
}
This works really well, but takes 3-5 seconds to load table which is fine but I am looking for ways to improve this data fetch .

Answer 1

Realm works on a principle of lazy-loading, where objects and their properties aren't loaded until you actually 'touch' them for the first time.
As a result, if you do any operations where you're manually iterating through all Realm objects in a results set at once, or manually copying specific objects to an array, you're going to incur a performance hit that will increase the more objects you persist in Realm.
The best way to minimize the performance hit is to try and mitigate how many times you iterate through the results sets and avoid copying objects out of the array as much as possible. RLMResults behaves like an array, so for most scenarios, you can usually just use that object instead.
In the prepareDataSource method, instead of looping through each object and passing them to that NSMutableArray, instead you could consider passing the RLMResults object itself instead.
The method getSectionTitleBasedOn: also seems quite inefficient since you're iterating through every single object in order to check if an entry with a particular first character exists. Instead, you could create an index of the alphabet, and then do a Realm query for entries that start with each letter, and then check to see if the resulting RLMResults object has a positive count (Though I'm not sure if this will actually be any faster).
But in the end, sometimes when you're doing complex sorting like this, where there's no 'clever' way to avoid iterating through each object in a database (Even Realm has to internally load each object when performing a sort), performance hits are unavoidable, in which case you should also make sure your UI has provisions to show a 'working' indicator to the user.