Unexpectedly large Realm file size

Question

This question is about using two different ways to insert objects into a Realm. I noticed that the first method is a lot faster, but the size result is huge comparing with t

Answer 1

You should in most cases try to minimize the number of write transactions. A write transaction has a significant overhead, hence if you start a new write transaction for every object you want to add to realm, your code will be significantly slower than if you added all objects using a single write transaction.

In my experience, the best way to add several elements to realm is to create the elements, add them to an array and then add the array as a whole to Realm using a single write transaction.

So this is what you should be doing:

var objects = [realmObj]()
for i in 1...40000{
    let newRealmObj = realmObj(value: ["id" : incrementID(), "a": "123","b": 12.12,"c": 66,"d": 13.13,"e": 0.6,"f": "01100110","g": DateTime, "h": 3])
    objects.append(newRealmObj)
}
try! realm.write {
    realm.add(objects)
}

As for the size issue, see the Limitations - File Size part of the Realm documentation. I am not 100% sure on the cause of the issue, but I would say that the issue is caused by writing code inside the write transaction that doesn't need to happen there and shouldn't happen inside the write transaction. I guess due to this, Realm creates a lot of intermediate versions of your objects and since releasing reserved storage capacity is quite an expensive operation, it doesn't happen by the time you are checking the file size.

Keep in mind, that the creation of objects doesn't need to happen inside a write transaction. You only need to create a write transaction for modifying persisted data in Realm (which includes adding new objects to Realm, deleting persisted objects and modifying persisted objects directly).