Store images in sqlite or just a reference to it?

问题:

I have made couple of apps using coredata and I was storing images in sqlite, but somewhere i found that it is bad. I've searched the net but all I've found is this suggestion:

• image size < 100kb store in the same table as the relevant data
• image size < 1mb store in a separate table attached via a relationship to avoid loading unnecessarily
• image size > 1mb store on disk and reference it inside of Core Data

So my question is: what are pros and cons of saving an image in sqlite db as NSData, and storing just a reference to the image while image is saved in the file system?

回答1:

Apple provide some guidance on this topic in their guide on Core Data Performance. In general, although SQLite scales pretty well and can handle databases that are many gigabytes in size with ease, large binary blobs are not queryable or indexable, and inflate the size of the database with little return.

If you're targeting iOS 4 and above, you can set the "Allows External Binary Data Storage" flag on your attributes that contain such data, and Core Data will automatically store them separately on the file system (if it deems appropriate), and automatically manage the link to that data in your data store.

回答2:

Benefits: Not so sure, but I can think of couple of benefits of storing just links in the database.

1. The native code interaction with the file system would be faster than the SQLite image fetching. (overall faster performance)

2. Clean and scalable database -- (with size being the concern, migration would be easier)

回答3:

You may want to check the answer I get for a similiar, if not the same, topic. Because as you I only found person giving advices, but no one really was providing benchmark and real technical answer.

Provide example for why it is not advisable to store images in CoreData?

Beside that, after my app have been realized with all images in db, and shipped to app store, I can tell you that things are easier if you use iCloud. If you use small images in UITableView with thumbnail icons, you can completely avoid asynchronous image loading.

Only one advice, provide an entity for each images size, rather storing all in a set attached to main entity.

The only downside I found, with iCloud use, is the larger transaction log generated each time I change an image. But in my case image are small, and the needs of updating the images is rare. Also, iCloud+CoreData at the moment is quite buggy so I removed it before shipping, so at the moment it is really not a problem for me.