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POThe performance of NSSortDescriptor?
 

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  1. POThe performance of NSSortDescriptor?
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    <p>I have a SQLite backed database of roughly 4000 rows, connected to Core Data. The model is a simple overview -> detail model. Each overview (with title and subtitle) has a relationship to a detail, which contains detailed information. To view this data I have implemented a UITableView with a NSFetchedRequestController.</p> <pre><code>NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"registered_name" ascending:YES]; NSArray *sortDescriptors = [NSArray arrayWithObject:sort]; NSPredicate *filterPredicate = nil; NSString *sectionName = nil; NSString *cacheName = nil; NSFetchedResultsController *aFetchedResultsController = nil; aFetchedResultsController.delegate = self; NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; [fetchRequest setFetchBatchSize:20]; [fetchRequest setSortDescriptors:sortDescriptors]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Info" inManagedObjectContext:self.managedObjectContext]; [fetchRequest setEntity:entity]; [fetchRequest setPredicate:filterPredicate]; aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:sectionName cacheName:cacheName]; NSError *error = nil; if (![aFetchedResultsController performFetch:&amp;error]) NSLog(@"Unresolved error %@, %@", error, [error userInfo]); abort(); } [fetchRequest release]; [sort release]; return aFetchedResultsController;` </code></pre> <p>I have implemented everything and it works, but it works slowly. Specifically I'm concerned with the application launch.</p> <p>This is the query that slows things down on launch time (SQL debug flag in XCode):</p> <pre><code>CoreData: sql: SELECT 0, t0.Z_PK FROM ZINFO t0 ORDER BY t0.ZREGISTERED_NAME CoreData: annotation: sql connection fetch time: 0.8773s CoreData: annotation: total fetch execution time: 0.8976s for 4302 rows. </code></pre> <p>This happens immediately after performFetch is executed. Why does Core Data feel the need to get and sort all 4302 rows of my database? If I set the setLimit to a small value, say 20, of course everything works extremely fast, but I only get 20 results displayed. On the other hand, I now have setBatchSize, which works great, and I can see the SELECT statements in the debug console, as I scroll the table view. But it still sorts 4302 rows at launch, which I feel is unnecessarily slowing down app launch time.</p> <p>I intend to implement a normalized string sorting and a caseInsensitiveNumericCompare selector in NSSortDescriptor, as per Apple's WWDC 2010 sample code, but it seems to me I'm missing something simple here.</p> <h1>Update:</h1> <p>Seems that I can't use a custom selector caseInsensitiveNumericCompare: with NSSortDescriptor, since I am using SQLite as the database. I really don't see what more I can do, to speed this up. Maybe SQLite will sort faster with shortened and normalized string, this is what I'm trying next.</p> <h1>Update 2:</h1> <p>With normalized string (no other letters or symbols except A-Z and 0-9), the launch time is down to about 0.7s. Not a big reduction. The last thing I'm trying is presorting the database and then assigning incremental ids to the rows. In NSSortDescriptor I will then sort by this numerical id. According to my SQL tests, it should be roughly 7x faster. </p>
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Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

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