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Re: [IndexedDB] IDBCursor.update for cursors returned from IDBIndex.openCursor

From: Andrei Popescu <andreip@google.com>
Date: Thu, 16 Sep 2010 12:24:50 +0100
Message-ID: <AANLkTimLXVnM_x8EvCcR-74Lqxnjak94Kzb7sS1EA3rP@mail.gmail.com>
To: Jeremy Orlow <jorlow@chromium.org>
Cc: Jonas Sicking <jonas@sicking.cc>, public-webapps WG <public-webapps@w3.org>
On Thu, Sep 16, 2010 at 10:15 AM, Jeremy Orlow <jorlow@chromium.org> wrote:
> On Wed, Sep 15, 2010 at 10:45 PM, Jonas Sicking <jonas@sicking.cc> wrote:
>>
>> Heh, I've also been thinking about this exact issue lately. There is a
>> similar question for IDBCursor.delete.
>>
>> On Wed, Sep 15, 2010 at 8:55 AM, Jeremy Orlow <jorlow@chromium.org> wrote:
>> > I think it's clear what IDBCursor does when created from
>> > IDBObjectStore.openCursor or IDBIndex.openObjectCursor: it modifies the
>> > objectStore's value and updates all indexes (or does nothing and returns
>> > an
>> > error if all of that can't be done while satisfying the constraints).
>>
>> Agreed.
>>
>> > But what about IDBCursor.update when created from IDBIndex.openCursor?
>> >  I
>> > see two options: we could modify the value within the objectStore's
>> > value
>> > that corresponds to the objectStore's key path or we could do like above
>> > and
>> > simply modify the objectStore's value.
>>
>> There's also a third option: Throw an exception. Maybe that's what
>> you're referring to by "make this unsupported" below?
>>
>> > More concretely, if we have an object store with a "id" key path and an
>> > index with a "fname" key path, and our index.openCursor() created cursor
>> > is
>> > currently on the {id: 22, fname: "Fred"} value (and thus cursor.key ==
>> > "Fred" and cursor.value == 22), let's say I wanted to change the object
>> > to
>> > be {id: 23, fname: "Fred"}.  In other words, I want id to change from 22
>> > to
>> > 23.  Which of the following should I write?
>> > 1) calling cursor.update(23)   or
>> > 2) calling cursor.update({id: 23, fname: "Fred"})
>> > The former seems to match the behavior of the IDBObjectStore.openCursor
>> > and
>> > IDBIndex.openObjectCursor better (i.e. it modifies the cursor.value).
>> >  The
>> > latter intuitively seems like it'd be more useful.  But to be honest, I
>> > can't think of any use cases for either.  Can anyone else?  If not,
>> > maybe we
>> > should just make this unsupported for now?
>>
>> The only use case I have thought of is wanting to update some set of
>> entries, where the best way to find these entries is through an index.
>> For example updating every entry with a specific shipping-id. You can
>> use IDBIndex.openObjectCursor for this, but that's slower than
>> IDBIndex.openCursor. So in the rare instance when you can make the
>> modification without inspecting the existing value (i.e. you only need
>> to write, read-modify-write), then IDBIndex.openCursor +
>> IDBCursor.update() would be a perf optimization.
>>
>> On the other hand, it might be just as quick to call IDBObjectStore.put().
>>
>> Since the use case if pretty weak (when would you be able to update an
>> entry without first reading the entry), and that you can seemingly get
>> the same performance using IDBObjectStore.put(), I would be fine with
>> making this unsupported.
>>
>> As for IDBCursor.delete(), I can see a somewhat stronger use case
>> there. For example removing all entries with a specific shipping-id or
>> some such. If you can determine which entries should be removed purely
>> on the information in the index, then using IDBIndex.openCursor is
>> definitely faster than IDBIndex.openObjectCursor. So on one hand it
>> would be nice to allow people to use that. On the other hand, I
>> suspect you can get the same performance using IDBObjectStore.delete()
>> and we might want to be consistent with IDBCursor.update().
>>
>> In this case I'm actually leaning towards allowing IDBCursor.delete(),
>> but I could go either way.
>
> Wait a sec.  What are the use cases for non-object cursors anyway?  They
> made perfect sense back when we allowed explicit index management, but now
> they kind of seem like a premature optimization or possibly even dead
> weight.  Maybe we should just remove them altogether?

I guess the reason for having non-object cursors is just performance:
it's probably faster to iterate a non-object cursor since you're only
iterating over the primary keys of the records in the object store and
not over the full records. But I can't really come up with a
convincing usecase to justify this. I think it's fine to remove them.

Andrei
Received on Thursday, 16 September 2010 11:25:22 GMT

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