Core Data VS Sqlite or FMDB....?

Question

Now this might look like a duplicate thread, but my question is that I have read a lot of questions like.. Core Data vs SQLite 3 and others but these are 2-3 years old. I have also read that FMDB was developed as core data was not supported on iOS, So it should not be used any more. And on the other hand I have read that one should not use core data as a database.

So I am seriously confused,whether I should use core data for object storage or not. I mean on what basis I should decide which to use? Are there any guidelines provided by apple or someone else.. or is it something that will come to me with time.?

Answer 1

Ankit,

Here's the tl;dr skinny: use Core Data.

Here's the long form:

While you could use many criteria to choose between Core Data, an ORM (FMDB) or direct sqlite calls, the real cost of this choice comes from your time to use it, Apple's support and leverage from other projects. (RESTKit, which maps REST services on to Core Data, is popular these days.)

Hence, a large percentage of the time, say 90+% (a made up stat), the answer on iOS will be to use Core Data. Why? Once you get the hang of it and build out a few little helper methods, Core Data keeps you in a consistent computing world -- the Objective-C object graph. Core Data will teach you things about how to use a dynamic language that will help every other aspect of your iOS programming. Hence, you are more productive. Don't fight the framework.

If you are bringing over a large, complex SQLite database & schema from another app, it then might be cost effective to use either FMDB or SQLite. But I doubt it. Your time writing a simple Mac-based command line app to migrate the DB to a Core Data DB is a finite and simple task. You are almost guaranteed to have to rewrite most of the business logic in Objective-C. (Yes, C++ and Objective-C++ are both good technologies. Has your database business logic really been tuned to work on a memory limited device? I didn't think so.)

Core Data gets a bum rap on performance. It is really quite fast. You just have to use it differently than you use a DB. In particular, you almost always over-fetch data from the store and then refine it using predicates directly on the various sets and arrays. On iOS devices, where the flash is surprisingly slow, this over-fetch strategy is particularly effective. You actually have a lot of RAM on these devices, use it to gain performance. (Yes, I know this is an apparent contradiction to my above knock on portable business logic. But really, code ported from a desktop or server environment has so many implicit assumptions about the speed of the disk, the amount of memory and the reality of a VM with a backing store, it just will not work well on a battery powered, memory limited device with a funky memory model. [It won't work very well on Android devices either.]) You will also denormalize your data to simplify displaying it in various iOS and Mac OS X UI widgets. There are a few applications where Core Data will be slower than an equivalent SQLite DB. Those have been detailed elsewhere. The one major claim is that tasks where IDs are defined by upstream databases hits Core Data's performance is true. But it can be somewhat mitigated by judicious indexing and over-fetching.

The thing to remember about mobile devices too is that the database size, because these are mobile devices on the leaves of the internet, is generally of modest size. Hence, performance is easier to attain. Many lessons from the world of servers may not apply to this mobile, battery powered world.

In other words, you've had to go "all in" to use Objective-C on iOS/Mac OS X, you will gain some important productivity benefits from using Core Data too.

Andrew

Answer 2

I recently embarked on this journey myself, and ended up trying out all three. Here's what I learned:

• Raw sqlite3
  • Low-level, full access to database. No abstractions. Very verbose - it takes a good deal of code to do very simple things.
• Core Data
  • Very high-level, built on abstractions, MUST use Apple's generated database. Useful for iCloud synchronization and simple iOS-only data management. Difficult and dangerous to directly access database, and should not be used for cross-platform databases. Still takes a fair amount of code to do simple things.
• FMDB
  • High-level, very abstraction friendly but not forced. Still get full access to database if you need it. Provides an NSDictionary of the result with each item automatically typecasted to the mutable variant of the proper datatype (e.g., text columns are returned as NSMutableString). I ended up building a very simple wrapper class around it to abstract it even more, so I have a helper class with static functions like selectAllFrom:(NSString *)table where:(NSDictionary *)conditions, which returns an NSArray of NSDictionary objects. It's fantastic to be able to do things like NSArray *usersNamedJoe = [DBHelper selectAllFrom:@"user" where:@{@"name": @"Joe"}];.

Basically, while Core Data may be useful for simple iOS-only apps, anyone who's interested in using cross-platform databases should stay far, far away from it - Apple has no interest in making that easy, and it shows.

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TL;DR:

• Don't use raw sqlite3 unless you're doing something extremely trivial.
• Core Data is fine for simple iOS-only data, if you're comfortable with being locked into it.
• If you want full control over the database and you're not doing something trivial, or you're building your app for multiple platforms, FMDB is definitely the way to go.