Slow save after changing Core Data relationship validation rule

Question

Created Jul ’16

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In Xcode on my data model, I created a new version and just modified a relationship to be optional (instead of non-optional).

Running my updated app with a large database created using previous model version will cause the first save to run really slow. This happens after each app launch!

If I build a large database from scratch using my updated app (means, using new model version) there will be no issue.

Note that I modified the relationship on an entity holding an image data (blob).

Any thoughts?

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Answer 1

Yoash OP

Jul ’16

UPDATE 1:

after adding

-com.apple.CoreData.SQLDebug 1

I figured the following:

Following the added model version, there is a lightweight migration that rebuilds the image table and re-generate all indexes.

On the first save this what happens:

```
BEGIN EXCLUSIVE
```
few
```
UPDATE
```
```
COMMIT
```
(very fast!)
```
pragma page_count
```
```
pragma freelist_count
```
```
pragma incremental_vacuum(4055)
```
(very slow, almost 4 seconds!)

pragma incremental_vacuum

is taking almost 4 seconds on iPhone 6 device and much more time on slower devices (more than 10 seconds).

Note that on the first save, no matter which app you develop, steps 1-5 will always take place after app loaded.

Unique for my case is that I get

incremental_vacuum

in addition. I can see why it's invoked (I guess because there are too many unused pages due to rebuilding the large image table in the lightweight migration phase), however, I can't see why it's invoked later on subsequent app loads.

Shouldn't the first invokation of

incremental_vacuum

fix the issue of unused page and update the sqlite file accordingly? It just seems that the result of

incremental_vacuum

is kept in memory and not saved to the file (although file is indeed saved)

0