@Kenn / @Rob has there been any other discussions on how the timestamp
value can be accessed from within the SQL since this thread in May?
If not my vote is for a convenience method that gives access to the
timestamp as a function call within the SQL statement.
Reza
On Wed, 22 May 2019 at 10:06, Reza Rokni <r...@google.com> wrote:
> Hi,
>
> Coming back to this do we have enough of a consensus to say that in
> principle this is a good idea? If yes I will raise a Jira for this.
>
> Cheers
>
> Reza
>
> On Thu, 16 May 2019 at 02:58, Robert Bradshaw <rober...@google.com> wrote:
>
>> On Wed, May 15, 2019 at 8:51 PM Kenneth Knowles <k...@apache.org> wrote:
>> >
>> > On Wed, May 15, 2019 at 3:05 AM Robert Bradshaw <rober...@google.com>
>> wrote:
>> >>
>> >> Isn't there an API for concisely computing new fields from old ones?
>> >> Perhaps these expressions could contain references to metadata value
>> >> such as timestamp. Otherwise,
>> >
>> > Even so, being able to refer to the timestamp implies something about
>> its presence in a namespace, shared with other user-decided names.
>>
>> I was thinking that functions may live in a different namespace than
>> fields.
>>
>> > And it may be nice for users to use that API within the composite
>> SqlTransform. I think there are a lot of options.
>> >
>> >> Rather than withMetadata reifying the value as a nested field, with
>> >> the timestamp, window, etc. at the top level, one could let it take a
>> >> field name argument that attaches all the metadata as an extra
>> >> (struct-like) field. This would be like attachX, but without having to
>> >> have a separate method for every X.
>> >
>> > If you leave the input field names at the top level, then any "attach"
>> style API requires choosing a name that doesn't conflict with input field
>> names. You can't write a generic transform that works with all inputs. I
>> think it is much simpler to move the input field all into a nested
>> row/struct. Putting all the metadata in a second nested row/struct is just
>> as good as top-level, perhaps. But moving the input into the struct/row is
>> important.
>>
>> Very good point about writing generic transforms. It does mean a lot
>> of editing if one decides one wants to access the metadata field(s)
>> after-the-fact. (I also don't think we need to put the metadata in a
>> nested struct if the value is.)
>>
>> >> It seems restrictive to only consider this a a special mode for
>> >> SqlTransform rather than a more generic operation. (For SQL, my first
>> >> instinct would be to just make this a special function like
>> >> element_timestamp(), but there is some ambiguity there when there are
>> >> multiple tables in the expression.)
>> >
>> > I would propose it as both: we already have some Reify transforms, and
>> you could make a general operation that does this small data preparation
>> easily. I think the proposal is just to add a convenience build method on
>> SqlTransform to include the underlying functionality as part of the
>> composite, which we really already have.
>> >
>> > I don't think we should extend SQL with built-in functions for
>> element_timestamp() and things like that, because SQL already has TIMESTAMP
>> columns and it is very natural to use SQL on unbounded relations where the
>> timestamp is just part of the data.
>>
>> That's why I was suggesting a single element_metadata() rather than
>> exploding each one out.
>>
>> Do you have a pointer to what the TIMESTAMP columns are? (I'm assuming
>> this is a special field, but distinct from the metadata timestamp?)
>>
>> >> On Wed, May 15, 2019 at 5:03 AM Reza Rokni <r...@google.com> wrote:
>> >> >
>> >> > Hi,
>> >> >
>> >> > One use case would be when dealing with the windowing functions for
>> example:
>> >> >
>> >> > SELECT f_int, COUNT(*) , TUMBLE_START(f_timestamp, INTERVAL '1'
>> HOUR) tumble_start
>> >> > FROM PCOLLECTION
>> >> > GROUP BY
>> >> > f_int,
>> >> > TUMBLE(f_timestamp, INTERVAL '1' HOUR)
>> >> >
>> >> > For an element which is using Metadata to inform the EvenTime of the
>> element, rather than data within the element itself, I would need to create
>> a new schema which added the timestamp as a field. I think other examples
>> which maybe interesting is getting the value of a row with the max/min
>> timestamp. None of this would be difficult but it does feel a little on the
>> verbose side and also makes the pipeline a little harder to read.
>> >> >
>> >> > Cheers
>> >> > Reza
>> >> >
>> >> >
>> >> >
>> >> >
>> >> >
>> >> > From: Kenneth Knowles <k...@apache.org>
>> >> > Date: Wed, 15 May 2019 at 01:15
>> >> > To: dev
>> >> >
>> >> >> We have support for nested rows so this should be easy. The
>> .withMetadata would reify the struct, moving from Row to WindowedValue<Row>
>> if I understand it...
>> >> >>
>> >> >> SqlTransform.query("SELECT field1 from PCOLLECTION"):
>> >> >>
>> >> >> Schema = {
>> >> >> field1: type1,
>> >> >> field2: type2
>> >> >> }
>> >> >>
>> >> >> SqlTransform.query(...)
>> >> >>
>> >> >> SqlTransform.withMetadata().query("SELECT event_timestamp,
>> value.field1 FROM PCOLLECTION")
>> >> >>
>> >> >> Derived schema = {
>> >> >> event_timestamp: TIMESTAMP,
>> >> >> pane_info: { ... }
>> >> >> value: {
>> >> >> field1: type1,
>> >> >> field2: type2,
>> >> >> ...
>> >> >> }
>> >> >> }
>> >> >>
>> >> >> SqlTransform would expand into a different composite, and it would
>> be a straightforward ParDo to adjust the data, possibly automatic via the
>> new schema conversions.
>> >> >>
>> >> >> Embedding the window would be a bit wonky, something like {
>> end_of_window: TIMESTAMP, encoded_window: bytes } which would be expensive
>> due to encoding. But timestamp and pane info not so bad.
>> >> >>
>> >> >> Kenn
>> >> >>
>> >> >> From: Anton Kedin <ke...@google.com>
>> >> >> Date: Tue, May 14, 2019 at 9:17 AM
>> >> >> To: <dev@beam.apache.org>
>> >> >>
>> >> >>> Reza, can you share more thoughts on how you think this can work
>> end-to-end?
>> >> >>>
>> >> >>> Currently the approach is that populating the rows with the data
>> happens before the SqlTransform, and within the query you can only use the
>> things that are already in the rows or in the catalog/schema (or built-in
>> things). In general case populating the rows with any data can be solved
>> via a ParDo before SqlTransform. Do you think this approach lacks something
>> or maybe too verbose?
>> >> >>>
>> >> >>> My thoughts on this, lacking more info or concrete examples: in
>> order to access a timestamp value from within a query there has to be a
>> syntax for it. Field access expressions or function calls are the only
>> things that come to mind among existing syntax features that would allow
>> that. Making timestamp a field of the data row makes more sense to me here
>> because in Beam it is already a part of the element. It's not a result of a
>> function call and it's already easily accessible, doesn't make sense to
>> build extra functions here. One of the problems with both approaches
>> however is the potential conflicts with the existing schema of the data
>> elements (or the schema/catalog of the data source in general). E.g. if we
>> add a magical "event_timestamp" column or "event_timestamp()" function
>> there may potentially already exist a field or a function in the schema
>> with this name. This can be solved in couple of ways, but we will probably
>> want to provide a configuration mechanism to assign a different
>> field/function names in case of conflicts.
>> >> >>>
>> >> >>> Given that, it may make sense to allow users to attach the whole
>> pane info or some subset of it to the row (e.g. only the timestamp), and
>> make that configurable. However I am not sure whether exposing something
>> like pane info is enough and will cover a lot of useful cases. Plus adding
>> methods like `attachTimestamp("fieldname")` or
>> `attachWindowInfo("fieldname")` might open a portal to ever-increasing
>> collection of these `attachX()`, `attachY()` that can make the API less
>> usable. If on the other hand we would make it more generic then it will
>> probably have to look a lot like a ParDo or MapElements.via() anyway. And
>> at that point the question would be whether it makes sense to build
>> something extra that probably looks and functions like an existing feature.
>> >> >>>
>> >> >>> Regards,
>> >> >>> Anton
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>> From: Andrew Pilloud <apill...@google.com>
>> >> >>> Date: Tue, May 14, 2019 at 7:29 AM
>> >> >>> To: dev
>> >> >>>
>> >> >>>> Hi Reza,
>> >> >>>>
>> >> >>>> Where will this metadata be coming from? Beam SQL is tightly
>> coupled with the schema of the PCollection, so adding fields not in the
>> data would be difficult.
>> >> >>>>
>> >> >>>> If what you want is the timestamp out of the DoFn.ProcessContext
>> we might be able to add a SQL function to fetch that.
>> >> >>>>
>> >> >>>> Andrew
>> >> >>>>
>> >> >>>> From: Reza Rokni <r...@google.com>
>> >> >>>> Date: Tue, May 14, 2019, 1:08 AM
>> >> >>>> To: <dev@beam.apache.org>
>> >> >>>>
>> >> >>>>> Hi,
>> >> >>>>>
>> >> >>>>> What are folks thoughts about adding something like
>> SqlTransform.withMetadata().query(...)to enable users to be able to access
>> things like Timestamp information from within the query without having to
>> refiy the information into the element itself?
>> >> >>>>>
>> >> >>>>> Cheers
>> >> >>>>> Reza
>> >> >>>>>
>> >> >>>>>
>> >> >>>>>
>> >> >>>>> --
>> >> >>>>>
>> >> >>>>> This email may be confidential and privileged. If you received
>> this communication by mistake, please don't forward it to anyone else,
>> please erase all copies and attachments, and please let me know that it has
>> gone to the wrong person.
>> >> >>>>>
>> >> >>>>> The above terms reflect a potential business arrangement, are
>> provided solely as a basis for further discussion, and are not intended to
>> be and do not constitute a legally binding obligation. No legally binding
>> obligations will be created, implied, or inferred until an agreement in
>> final form is executed in writing by all parties involved.
>> >> >
>> >> >
>> >> >
>> >> > --
>> >> >
>> >> > This email may be confidential and privileged. If you received this
>> communication by mistake, please don't forward it to anyone else, please
>> erase all copies and attachments, and please let me know that it has gone
>> to the wrong person.
>> >> >
>> >> > The above terms reflect a potential business arrangement, are
>> provided solely as a basis for further discussion, and are not intended to
>> be and do not constitute a legally binding obligation. No legally binding
>> obligations will be created, implied, or inferred until an agreement in
>> final form is executed in writing by all parties involved.
>>
>
>
> --
>
> This email may be confidential and privileged. If you received this
> communication by mistake, please don't forward it to anyone else, please
> erase all copies and attachments, and please let me know that it has gone
> to the wrong person.
>
> The above terms reflect a potential business arrangement, are provided
> solely as a basis for further discussion, and are not intended to be and do
> not constitute a legally binding obligation. No legally binding obligations
> will be created, implied, or inferred until an agreement in final form is
> executed in writing by all parties involved.
>
--
This email may be confidential and privileged. If you received this
communication by mistake, please don't forward it to anyone else, please
erase all copies and attachments, and please let me know that it has gone
to the wrong person.
The above terms reflect a potential business arrangement, are provided
solely as a basis for further discussion, and are not intended to be and do
not constitute a legally binding obligation. No legally binding obligations
will be created, implied, or inferred until an agreement in final form is
executed in writing by all parties involved.
