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Re: Indexed Database API

From: Jonas Sicking <jonas@sicking.cc>
Date: Fri, 4 Mar 2011 17:36:35 -0800
Message-ID: <AANLkTimpk2BFyZ-K3auX2-LaGdqpxkwC0xYtuxetHsBc@mail.gmail.com>
To: Jeremy Orlow <jorlow@chromium.org>
Cc: ben turner <bent.mozilla@gmail.com>, Olli@pettay.fi, public-webapps@w3.org
A few observations:

1. It seems like a fairly rare use case to have to jump to item #100
without first also observing item 1-99. When showing a paged view
which lets the user to jump directly to, say, page 5 it can certainly
happen, but the page could optimize for the case when the user first
goes through page 1-4.
2. Since it's not a common case, adding support for it just on
cursors, rather than cursors and objectStores, seems enough. Would be
as simple as adding a .advance (or similarly named function) which
simply takes an integer. I don't see that we need to support jumping
in a arbitrary direction since we don't allow continue() in an
arbitrary direction.
3. We do have a bit of a hole in our index-cursor API. While you can
start the cursor at an arbitrary key, you can only start it at the
first entry with that key in the case when there are duplicate keys.
So if you iterate an index 10 records at a time, even if you never
need to skip any entries, you can't always resume where you left off,
even if you know the exact key+primaryKey for the record you want to
resume at.
4. While I agree that count() seems like a useful function, my concern
is that people might think it's a cheap operation. Getting the count
for a full objectStore or index should be quick, but getting the count
for a given key range (such as on a cursor) seems like it could be
expensive. My b-tree knowledge isn't the best, but isn't there a risk
that you have to linearly walk the full keyrange? Or does b-trees keep
an exact count of record in each node? Even if linear walking is
required, there might not be much we can do, and the best we can do is
to document that this is a slow operation.

/ Jonas

On Fri, Mar 4, 2011 at 2:32 PM, Jeremy Orlow <jorlow@chromium.org> wrote:
> On Fri, Mar 4, 2011 at 1:38 PM, ben turner <bent.mozilla@gmail.com> wrote:
>> Firefox does lazily deserialize cursor values, so the slowdown you're
>> noticing is most likely due to us preserving the order of request
>> callbacks by queuing every continue() call in line with the rest of
>> the transaction. Jonas had proposed a faster, high performance cursor
>> that did not respect this ordering, maybe that's all that you'd need.
>> However, a few thoughts:
>> 1. How do you know Page 5 even exists? We haven't exposed a count()
>> function yet...
>> 2. I think we should expose a count() function!
>> 3. Maybe we should expose a getAt(long index, <enum> direction);
>> function on indexes and objectStores?
> A count function might make sense.
> But in this case, you could just jump forward to page 5 and see if you get
> an error or not.
> I'd lean towards just adding jumping forward to cursors for now though.  If
> getting a single item at some position is popular, then we can always add
> it.
> Let's avoid adding prioritization of cursor.continue calls unless we have
> absolutely no other choice.
> J
>> On Fri, Mar 4, 2011 at 12:11 PM, Olli Pettay <Olli.Pettay@helsinki.fi>
>> wrote:
>> > On 03/02/2011 09:02 AM, Ben Dilts wrote:
>> >>
>> >> Why is there no mechanism for paging results, a la SQL's "limit"?  If I
>> >> want entries in positions 140-159 from an index, I have to call
>> >> continue() on a cursor 139 times, which in turn unserializes 139
>> >> objects
>> >> from my store that I don't care about, which in FF4 is making a lookup
>> >> in IndexedDB sometimes take many seconds for even a few records.
>> >
>> > Sounds like there is something to optimize in the implementation.
>> > Have you filed a bug
>> > https://bugzilla.mozilla.org/enter_bug.cgi?product=Core
>> > component DOM ?
>> > If not, please do so and attach a *minimal* testcase.
>> >
>> >
>> > Thanks,
>> >
>> >
>> > -Olli
>> >
>> >
>> >  It
>> >>
>> >> makes no sense--am I just missing something in the spec?
>> >>
>> >>
>> >> Ben Dilts
>> >
>> >
Received on Saturday, 5 March 2011 01:37:38 UTC

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