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Re: Counting instances in Core Data
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Re: Counting instances in Core Data


  • Subject: Re: Counting instances in Core Data
  • From: Adam Swift <email@hidden>
  • Date: Tue, 29 Apr 2008 11:33:27 -0700


On Apr 29, 2008, at 11:18 AM, Adam Swift wrote:


On Apr 29, 2008, at 9:03 AM, Steve Cronin wrote:

My question is: what is the most efficient fetch to pose given that every fetch is IO.
A given entity might have a lot of records so an array COULD be an unnecessarily large transaction...


Is there a best practice fetch for this 'get a count' purpose?


Sorry - I should have pointed this out- there are a lot of options available for tuning performance (fetch IDs, prefetch relationships, fetch in batches, etc, etc) and it definitely makes sense to measure before you start optimizing -- this covers the breadth and summarizes it very nicely:


http://developer.apple.com/documentation/Cocoa/Conceptual/CoreData/Articles/cdPerformance.html#/ /apple_ref/doc/uid/TP40003468

In Tiger that was the best means available, but if you're targeting Leopard there is a better option available:

Create your fetch request to fetch all instances of a given entity and execute the fetch with countForFetchRequest:error: which will simply return the number of instances/rows that match your query.

request = [[[NSFetchRequest alloc] init] autorelease];
[request setEntity:[NSEntityDescription entityForName:@"Foo" inManagedObjectContext:context]];
count = [context countForFetchRequest:request error:&error];


http://developer.apple.com/documentation/Cocoa/Reference/CoreDataFramework/Classes/NSManagedObjectContext_Class/Reference/Reference.html

- adam

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References: 
 >Counting instances in Core Data (From: Steve Cronin <email@hidden>)
 >Re: Counting instances in Core Data (From: Adam Swift <email@hidden>)

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