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Editorial Team
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Asked: 2026-06-06T21:50:30+00:00

I was curious if there was a performance gain from creating a Fetch Request

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I was curious if there was a performance gain from creating a Fetch Request Template versus programmatically creating NSFetchRequest so I wrote some tests to measure this. Here’s the source on github.

Same order of difference in Simulator and on iPhone (measuring time interval to do a bunch of fetches):

just creating an NSFetchRequest:          4.399674
creating a Fetch Request Template:        0.501369
NSFetchRequest with field indexed:        0.407068
Fetch Request Template and field indexed: 0.281876

It turns out there’s about 7~9 times performance gain in creating a Fetch Request Template. I thought maybe it’s creating the proper index underneath but when I created an Fetch Request Template that matches on an indexed field, there’s even a further performance benefit.

Ok, nice to know that this is the case, but I would very much like to know what the heck is happening underneath the Fetch Request Template that accounts for performance gains?

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1 Answer

  1. Editorial Team

    Update

    After some profiling with Instruments, it turns out [NSPredicate predicateWithFormat:] is not the culprit here!

    The actual cause of the performance difference is sort descriptors.

    The non-template tests are using an NSFetchedResultsController, which requires a sort descriptor, whereas the template-based ones do not specify a sort descriptor.

    If you add a sort descriptor to all the tests, the performance evens out (with the exception of the indexed case.)

    Original (wrong) answer

    The performance penalty is because your ‘just creating a NSFetchRequest’ test calls [NSPredicate predicateWithFormat:] for each iteration of the loop – this is very slow!

    Think about it – [NSPredicate predicateWithFormat:] has to parse the string and basically compile it into the internal representation used by Core Data.

    The usual solution is to only call [NSPredicate predicateWithFormat:] once, and then use [NSPredicate predicateWithSubstitutionVariables:] to specify the values that will be compared by the predicate – this is covered in the Core Data Documentation – Efficiently Importing Data

    To create a predicate from a formatted string, the framework must
    parse the string and create instances of predicate and expression
    objects. If you are using the same form of a predicate many times over
    but changing the value of one of the constant value expressions on
    each use, it is more efficient to create a predicate once and then use
    variable substitution (see “Creating Predicates”).

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