re:[Core Data] Difference between objectWithID: and a fetch "self IN %@", someObjectIDs ????
re:[Core Data] Difference between objectWithID: and a fetch "self IN %@", someObjectIDs ????
- Subject: re:[Core Data] Difference between objectWithID: and a fetch "self IN %@", someObjectIDs ????
- From: Ben Trumbull <email@hidden>
- Date: Wed, 20 May 2009 12:15:32 -0700
In the main thread, using objectWithID: repeatedly on each objectID
sent by the subthread returns the object that was inserted in the
background thread. This is exactly what I want.
But reading from the doc, it seems that we are encouraged to use a
fetch using "self IN %@",listOfObjectIDs instead of objectWithID:
since it is faster for many objects...
Unfortunately, this fetch returns 0 results (and yes this is the
correct entity, store and moc...)
The objects probably haven't been saved yet.
what does objectWithID: *exactly* do that a fetch does not ??
Imagine the NSPersistentStoreCoordinator has a cache of row data for
recently fetched or saved objects. This cache is used to fulfill
faults quickly. Faulting becomes very slow when it needs to go all
the way to the database due to a cache miss. The Core Data template
in Instruments shows this.
But cache hits are very fast. This is how most managed objects get
initialized.
Fetching always goes to disk and gets the latest values from the
database. Those values are then added to the PSC's cache.
-objectWithID: creates a fault. When you access the fault's data, it
first tries to populate the data from the PSC's cache. If it can't,
then it goes to disk.
The documentation is showing how to load a number of objects at once.
If the objects are unlikely to already be cached, then fetching all of
them in 1 transaction will be much faster than faulting them in one at
a time via -objectWithID:
- Ben
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