On Jun 23, 2008, at 17:11, Ian wrote:
Hi all,
Anyway, is there any way to query CoreData in chunks? I have a very
large CoreData SQl store (>1 million) of managed objects and it's
taking for ever (well, tens of minutes) to load it all in, never
mind to search through it all.
NSFetchRequest has a very handy setFetchLimit selector but it's
fairly useless (to my particular problem anyhow) without being able
to specify an offset.
Being able to say "give me the first 10000 rows" is cool and all,
but not being able to say "give me the NEXT 10000 rows" is a real
pain.
I know CoreData is supposed to be able to handle billions (citation: http://developer.apple.com/documentation/Cocoa/Conceptual/CoreData/Articles/cdPerformance.html
[search page text for "billions"]) of rows, but seriously, I'm not
getting these results. Any more than a few tens of thousands of
results and it's slow down city.
The problem is that Core Data doesn't use rows -- it doesn't have any
intrinsic ordering of objects (that's why unsorted results are always
sets, not arrays). There's no general concept of the "next" 10000
objects.
To query in chunks, you need to implement your own concept of ordering
(which may or may not be easy, depending on your requirements). For
example, if you can allocate each object a sequence number (as a
permanent attribute), you could use a sequence number range as part of
each potentially large fetch. If you turn on indexing for the
attribute, there should be no significant performance degradation
dependent on which range you fetch.
Obviously, a simple sequence number is not a good solution if many
objects can be randomly deleted. There's likely no good *general*
solution for all applications, which is (AFAIK) the reason Core Data
doesn't yet offer intrinsic ordering of objects. Another possibly
workable attribute might be date/time of object creation, date/time of
object modification, or a random hash of something like the object UID.
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