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From Dongjoon Hyun <dongjoon.h...@gmail.com>
Subject Re: [DISCUSS] SPIP: FunctionCatalog
Date Thu, 11 Feb 2021 02:06:20 GMT
BTW, I forgot to add my opinion explicitly in this thread because I was on
the PR before this thread.

1. The `FunctionCatalog API` PR was made on May 9, 2019 and has been there
for almost two years.
2. I already gave my +1 on that PR last Saturday because I agreed with the
latest updated design docs and AS-IS PR.

And, the rest of the progress in this thread is also very satisfying to me.
(e.g. Ryan's extension suggestion and Wenchen's alternative)

To All:
Please take a look at the design doc and the PR, and give us some opinions.
We really need your participation in order to make DSv2 more complete.
This will unblock other DSv2 features, too.

Bests,
Dongjoon.



On Wed, Feb 10, 2021 at 10:58 AM Dongjoon Hyun <dongjoon.hyun@gmail.com>
wrote:

> Hi, Ryan.
>
> We didn't move past anything (both yours and Wenchen's). What Wenchen
> suggested is double-checking the alternatives with the implementation to
> give more momentum to our discussion.
>
> Your new suggestion about optional extention also sounds like a new
> reasonable alternative to me.
>
> We are still discussing this topic together and I hope we can make a
> conclude at this time (for Apache Spark 3.2) without being stucked like
> last time.
>
> I really appreciate your leadership in this dicsussion and the moving
> direction of this discussion looks constructive to me. Let's give some time
> to the alternatives.
>
> Bests,
> Dongjoon.
>
> On Wed, Feb 10, 2021 at 10:14 AM Ryan Blue <blue@apache.org> wrote:
>
>> I don’t think we should so quickly move past the drawbacks of this
>> approach. The problems are significant enough that using invoke is not
>> sufficient on its own. But, I think we can add it as an optional extension
>> to shore up the weaknesses.
>>
>> Here’s a summary of the drawbacks:
>>
>>    - Magic function signatures are error-prone
>>    - Spark would need considerable code to help users find what went
>>    wrong
>>    - Spark would likely need to coerce arguments (e.g., String,
>>    Option[Int]) for usability
>>    - It is unclear how Spark will find the Java Method to call
>>    - Use cases that require varargs fall back to casting; users will
>>    also get this wrong (cast to String instead of UTF8String)
>>    - The non-codegen path is significantly slower
>>
>> The benefit of invoke is to avoid moving data into a row, like this:
>>
>> -- using invoke
>> int result = udfFunction(x, y)
>>
>> -- using row
>> udfRow.update(0, x); -- actual: values[0] = x;
>> udfRow.update(1, y);
>> int result = udfFunction(udfRow);
>>
>> And, again, that won’t actually help much in cases that require varargs.
>>
>> I suggest we add a new marker trait for BoundMethod called SupportsInvoke.
>> If that interface is implemented, then Spark will look for a method that
>> matches the expected signature based on the bound input type. If it isn’t
>> found, Spark can print a warning and fall back to the InternalRow call:
>> “Cannot find udfFunction(int, int)”.
>>
>> This approach allows the invoke optimization, but solves many of the
>> problems:
>>
>>    - The method to invoke is found using the proposed load and bind
>>    approach
>>    - Magic function signatures are optional and do not cause runtime
>>    failures
>>    - Because this is an optional optimization, Spark can be more strict
>>    about types
>>    - Varargs cases can still use rows
>>    - Non-codegen can use an evaluation method rather than falling back
>>    to slow Java reflection
>>
>> This seems like a good extension to me; this provides a plan for
>> optimizing the UDF call to avoid building a row, while the existing
>> proposal covers the other cases well and addresses how to locate these
>> function calls.
>>
>> This also highlights that the approach used in DSv2 and this proposal is
>> working: start small and use extensions to layer on more complex support.
>>
>> On Wed, Feb 10, 2021 at 9:04 AM Dongjoon Hyun <dongjoon.hyun@gmail.com>
>> wrote:
>>
>> Thank you all for making a giant move forward for Apache Spark 3.2.0.
>>> I'm really looking forward to seeing Wenchen's implementation.
>>> That would be greatly helpful to make a decision!
>>>
>>> > I'll implement my idea after the holiday and then we can have
>>> more effective discussions. We can also do benchmarks and get some real
>>> numbers.
>>> > FYI: the Presto UDF API
>>> <https://prestodb.io/docs/current/develop/functions.html> also
>>> takes individual parameters instead of the row parameter. I think this
>>> direction at least worth a try so that we can see the performance
>>> difference. It's also mentioned in the design doc as an alternative (Trino).
>>>
>>> Bests,
>>> Dongjoon.
>>>
>>>
>>> On Tue, Feb 9, 2021 at 10:18 PM Wenchen Fan <cloud0fan@gmail.com> wrote:
>>>
>>>> FYI: the Presto UDF API
>>>> <https://prestodb.io/docs/current/develop/functions.html> also
>>>> takes individual parameters instead of the row parameter. I think this
>>>> direction at least worth a try so that we can see the performance
>>>> difference. It's also mentioned in the design doc as an alternative (Trino).
>>>>
>>>> On Wed, Feb 10, 2021 at 10:18 AM Wenchen Fan <cloud0fan@gmail.com>
>>>> wrote:
>>>>
>>>>> Hi Holden,
>>>>>
>>>>> As Hyukjin said, following existing designs is not the principle of DS
>>>>> v2 API design. We should make sure the DS v2 API makes sense. AFAIK we
>>>>> didn't fully follow the catalog API design from Hive and I believe Ryan
>>>>> also agrees with it.
>>>>>
>>>>> I think the problem here is we were discussing some very detailed
>>>>> things without actual code. I'll implement my idea after the holiday
and
>>>>> then we can have more effective discussions. We can also do benchmarks
and
>>>>> get some real numbers.
>>>>>
>>>>> In the meantime, we can continue to discuss other parts of this
>>>>> proposal, and make a prototype if possible. Spark SQL has many active
>>>>> contributors/committers and this thread doesn't get much attention yet.
>>>>>
>>>>> On Wed, Feb 10, 2021 at 6:17 AM Hyukjin Kwon <gurwls223@gmail.com>
>>>>> wrote:
>>>>>
>>>>>> Just dropping a few lines. I remember that one of the goals in DSv2
>>>>>> is to correct the mistakes we made in the current Spark codes.
>>>>>> It would not have much point if we will happen to just follow and
>>>>>> mimic what Spark currently does. It might just end up with another
copy of
>>>>>> Spark APIs, e.g. Expression (internal) APIs. I sincerely would like
to
>>>>>> avoid this
>>>>>> I do believe we have been stuck mainly due to trying to come up with
>>>>>> a better design. We already have an ugly picture of the current Spark
APIs
>>>>>> to draw a better bigger picture.
>>>>>>
>>>>>>
>>>>>> 2021년 2월 10일 (수) 오전 3:28, Holden Karau <holden@pigscanfly.ca>님이
작성:
>>>>>>
>>>>>>> I think this proposal is a good set of trade-offs and has existed
in
>>>>>>> the community for a long period of time. I especially appreciate
how the
>>>>>>> design is focused on a minimal useful component, with future
optimizations
>>>>>>> considered from a point of view of making sure it's flexible,
but actual
>>>>>>> concrete decisions left for the future once we see how this API
is used. I
>>>>>>> think if we try and optimize everything right out of the gate,
we'll
>>>>>>> quickly get stuck (again) and not make any progress.
>>>>>>>
>>>>>>> On Mon, Feb 8, 2021 at 10:46 AM Ryan Blue <blue@apache.org>
wrote:
>>>>>>>
>>>>>>>> Hi everyone,
>>>>>>>>
>>>>>>>> I'd like to start a discussion for adding a FunctionCatalog
>>>>>>>> interface to catalog plugins. This will allow catalogs to
expose functions
>>>>>>>> to Spark, similar to how the TableCatalog interface allows
a catalog to
>>>>>>>> expose tables. The proposal doc is available here:
>>>>>>>> https://docs.google.com/document/d/1PLBieHIlxZjmoUB0ERF-VozCRJ0xw2j3qKvUNWpWA2U/edit
>>>>>>>>
>>>>>>>> Here's a high-level summary of some of the main design choices:
>>>>>>>> * Adds the ability to list and load functions, not to create
or
>>>>>>>> modify them in an external catalog
>>>>>>>> * Supports scalar, aggregate, and partial aggregate functions
>>>>>>>> * Uses load and bind steps for better error messages and
simpler
>>>>>>>> implementations
>>>>>>>> * Like the DSv2 table read and write APIs, it uses InternalRow
to
>>>>>>>> pass data
>>>>>>>> * Can be extended using mix-in interfaces to add vectorization,
>>>>>>>> codegen, and other future features
>>>>>>>>
>>>>>>>> There is also a PR with the proposed API:
>>>>>>>> https://github.com/apache/spark/pull/24559/files
>>>>>>>>
>>>>>>>> Let's discuss the proposal here rather than on that PR, to
get
>>>>>>>> better visibility. Also, please take the time to read the
proposal first.
>>>>>>>> That really helps clear up misconceptions.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>> Ryan Blue
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> Twitter: https://twitter.com/holdenkarau
>>>>>>> Books (Learning Spark, High Performance Spark, etc.):
>>>>>>> https://amzn.to/2MaRAG9  <https://amzn.to/2MaRAG9>
>>>>>>> YouTube Live Streams: https://www.youtube.com/user/holdenkarau
>>>>>>>
>>>>>> --
>> Ryan Blue
>>
>

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