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From Fabian Hueske <fhue...@gmail.com>
Subject Re: [DISCUSS] Table API / SQL indicators for event and processing time
Date Thu, 16 Feb 2017 00:18:35 GMT
Thanks everybody for the comments.

Actually, I think we do not have much choice when deciding whether to use
attributes or functions.
Consider the following join query:

SELECT​ ​t1.amount​,​ ​t2.rate
FROM​ ​
  table1 ​AS​ t1,
​ ​ table2 ​AS​ ​t2
WHERE ​
  t1.currency = t2.currency AND
  t2.rowtime ​=​ ​(
​ ​​ ​  SELECT​ ​MAX(t22.rowtime)
​ ​​ ​  FROM​ ​table2 ​AS​ t22
​ ​​   ​AND​ ​t22.rowtime ​<=​ t1.rowtime)

The query joins two streaming tables. Table 1 is a streaming table with
amounts in a certain currency. Table 2 is a (slowly changing) streaming
table of currency exchange rates.
We want to join the amounts stream with the exchange rate of the
corresponding currency that is valid (i.e., last received value ->
MAX(rowtime)) at the rowtime of the amounts row.
In order to specify the query, we need to refer to the rowtime of the
different tables. Hence, we need a way to relate the rowtime expression (or
marker) to a table.
This is not possible with a parameterless scalar function.

I'd like to comment on the concerns regarding the performance:
In fact, the columns could be completely virtual and only exist during
query parsing and validation.
During execution, we can directly access the rowtime metadata of a Flink
streaming record (which is present anyway) or look up the current
processing time from the machine clock. So the processing overhead would
actually be the same as with a marker function.

Regarding the question on what should be allowed with a system attribute:
IMO, it could be used as any other attribute. We need it at least in GROUP
BY, ORDER BY, and WHERE to define windows and joins. We could also allow to
access it in SELECT if we want users to give access to rowtime and
processing time. So @Haohui, your query could be supported.
However, what would not be allowed is to modify the value of the rows,
i.e., by naming another column rowtime, i.e., "SELECT sometimestamp AS
rowtime" would not be allowed, because Flink does not support to modify the
event time of a row (for good reasons) and processing time should not be
modifiable anyway.

@Timo:
I think the approach to only use the system columns during parsing and
validation and converting them to expressions afterwards makes a lot of
sense.
The question is how this approach could be nicely integrated with Calcite.

Best, Fabian


2017-02-15 16:50 GMT+01:00 Radu Tudoran <radu.tudoran@huawei.com>:

> Hi,
>
> My initial thought would be that it makes more sense to thave procTime()
> and rowTime() only as functions which in fact are to be used as markers.
> Having the value (even from special system attributes does not make sense
> in some scenario such as the ones for creating windows, e.g.,
> If you have SELECT Count(*) OVER (ORDER BY procTime()...)
> If you get the value of procTime you cannot do anything as you need the
> marker to know how to construct the window logic.
>
> However, your final idea of having " implement some rule/logic that
> translates the attributes to special RexNodes internally " I believe is
> good and gives a solution to both problems. One the one hand for those
> scenarios where you need the value you can access the value, while for
> others you can see the special type of the RexNode and use it as a marker.
>
> Regarding keeping this data in a table...i am not sure as you would say
> we  need to augment the data with two fields whether needed or not...this
> is nto necessary very efficient
>
>
> Dr. Radu Tudoran
> Senior Research Engineer - Big Data Expert
> IT R&D Division
>
>
> HUAWEI TECHNOLOGIES Duesseldorf GmbH
> European Research Center
> Riesstrasse 25, 80992 München
>
> E-mail: radu.tudoran@huawei.com
> Mobile: +49 15209084330
> Telephone: +49 891588344173
>
> HUAWEI TECHNOLOGIES Duesseldorf GmbH
> Hansaallee 205, 40549 Düsseldorf, Germany, www.huawei.com
> Registered Office: Düsseldorf, Register Court Düsseldorf, HRB 56063,
> Managing Director: Bo PENG, Wanzhou MENG, Lifang CHEN
> Sitz der Gesellschaft: Düsseldorf, Amtsgericht Düsseldorf, HRB 56063,
> Geschäftsführer: Bo PENG, Wanzhou MENG, Lifang CHEN
> This e-mail and its attachments contain confidential information from
> HUAWEI, which is intended only for the person or entity whose address is
> listed above. Any use of the information contained herein in any way
> (including, but not limited to, total or partial disclosure, reproduction,
> or dissemination) by persons other than the intended recipient(s) is
> prohibited. If you receive this e-mail in error, please notify the sender
> by phone or email immediately and delete it!
>
> -----Original Message-----
> From: Timo Walther [mailto:twalthr@apache.org]
> Sent: Wednesday, February 15, 2017 9:33 AM
> To: dev@flink.apache.org
> Subject: Re: [DISCUSS] Table API / SQL indicators for event and processing
> time
>
> Hi all,
>
> at first I also thought that built-in functions (rowtime() and
> proctime()) are the easiest solution. However, I think to be future-proof
> we should make them system attributes; esp. to relate them to a
> corresponding table in case of multiple tables. Logically they are
> attributes of each row, which is already done in Table API.
>
> I will ask on the Calcite ML if there is a good way for integrating system
> attributes. Right now, I would propose the following implementation:
>
> - we introduce a custom row type (extending RelDataType)
> - in a streaming environment every row has two attributes by default
> (rowtime and proctime)
> - we do not allow creating a row type with those attributes (this should
> already prevent `SELECT field AS rowtime FROM ...`)
> - we need to ensure that these attributes are not part of expansion like
> `SELECT * FROM ...`
> - implement some rule/logic that translates the attributes to special
> RexNodes internally, such that the opimizer does not modify these attributes
>
> What do you think?
>
> Regards,
> Timo
>
>
>
>
> Am 15/02/17 um 03:36 schrieb Xingcan Cui:
> > Hi all,
> >
> > thanks for this thread.
> >
> > @Fabian If I didn't miss the point, the main difference between the
> > two approaches is whether or not taking these time attributes as
> > common table fields that are directly available to users. Whatever,
> > these time attributes should be attached to records (right?), and the
> > discussion lies in whether give them public qualifiers like other
> > common fields or private qualifiers and related get/set methods.
> >
> > The former (system attributes) approach will be more compatible with
> > existing SQL read-only operations (e.g., select, join), but we need to
> > add restrictions on SQL modification operation (like what?). I think
> > there are no needs to forbid users modifying these attributes via
> > table APIs (like map function). Just inform them about these special
> > attribute names like system built in aggregator names in iteration.
> >
> > As for the built in function approach, I don't know if, for now, there
> > are functions applied on a single row (maybe the value access
> > functions like COMPOSITE.get(STRING)?). It seems that most of the
> > built in functions work for a single field or on columns and thus it
> > will be mountains of work if we want to add a new kind of function to
> > SQL. Maybe all existing operations should be modified to support it.
> >
> > All in all, if there are existing supports for single row function, I
> > prefer the built in function approach. Otherwise the system attributes
> > approach should be better. After all there are not so much
> > modification operations in SQL and maybe we can use alias to support
> > time attributes setting (just hypothesis, not sure if it's feasible).
> >
> > @Haohui I think the given query is valid if we add a aggregate
> > function to (PROCTIME()
> > - ROWTIME()) / 1000 and it should be executed efficiently.
> >
> > Best,
> > Xingcan
> >
> > On Wed, Feb 15, 2017 at 6:17 AM, Haohui Mai <ricetons@gmail.com> wrote:
> >
> >> Hi,
> >>
> >> Thanks for starting the discussion. I can see there are multiple
> >> trade-offs in these two approaches. One question I have is that to
> >> which extent Flink wants to open its APIs to allow users to access
> >> both processing and event time.
> >>
> >> Before we talk about joins, my understanding for the two approaches
> >> that you mentioned are essentially (1) treating the value of event /
> >> processing time as first-class fields for each row, (2) limiting the
> >> scope of time indicators to only specifying windows. Take the
> >> following query as an
> >> example:
> >>
> >> SELECT (PROCTIME() - ROWTIME()) / 1000 AS latency FROM table GROUP BY
> >> FLOOR(PROCTIME() TO MINUTES)
> >>
> >> There are several questions we can ask:
> >>
> >> (1) Is it a valid query?
> >> (2) How efficient the query will be?
> >>
> >> For this query I can see arguments from both sides. I think at the
> >> end of the day it really comes down to what Flink wants to support.
> >> After working on FLINK-5624 I'm more inclined to support the second
> >> approach (i.e., built-in functions). The main reason why is that the
> >> APIs of Flink are designed to separate times from the real payloads.
> >> It probably makes sense for the Table / SQL APIs to have the same
> designs.
> >>
> >> For joins I don't have a clear answer on top of my head. Flink
> >> requires two streams to be put in the same window before doing the
> >> joins. This is essentially a subset of what SQL can express. I don't
> >> know what would be the best approach here.
> >>
> >> Regards,
> >> Haohui
> >>
> >>
> >> On Tue, Feb 14, 2017 at 12:26 AM Fabian Hueske <fhueske@gmail.com>
> wrote:
> >>
> >>> Hi,
> >>>
> >>> It would as in the query I gave as an example before:
> >>>
> >>> SELECT
> >>>    a,
> >>>    SUM(b) OVER (PARTITION BY c ORDER BY proctime ROWS BETWEEN 2
> >>> PRECEDING AND CURRENT ROW) AS sumB, FROM myStream
> >>>
> >>> Here "proctime" would be a system attribute of the table "myStream".
> >>> The table would also have another system attribute called "rowtime"
> >>> which would be used to indicate event time semantics.
> >>> These attributes would always be present in tables which are derived
> >>> from streams.
> >>> Because we still require that streams have timestamps and watermarks
> >>> assigned (either by the StreamTableSource or the somewhere
> >>> downstream the DataStream program) when they are converted into a
> >>> table, there is no
> >> need
> >>> to register anything.
> >>>
> >>> Does that answer your questions?
> >>>
> >>> Best, Fabian
> >>>
> >>>
> >>>
> >>> 2017-02-14 2:04 GMT+01:00 Radu Tudoran <radu.tudoran@huawei.com>:
> >>>
> >>>> Hi Fabian,
> >>>>
> >>>> Thanks for starting the discussion. Before I give my thoughts on
> >>>> this
> >> can
> >>>> you please give some examples of how would you see option of using
> >>> "system
> >>>> attributes"?
> >>>> Do you use this when you register the stream as a table, do you use
> >>>> if when you call an SQL query, do you use it when you translate
> >>>> back a
> >> table
> >>>> to a stream / write it to a dynamic table?
> >>>>
> >>>> Dr. Radu Tudoran
> >>>> Senior Research Engineer - Big Data Expert IT R&D Division
> >>>>
> >>>>
> >>>> HUAWEI TECHNOLOGIES Duesseldorf GmbH
> >>>> European Research Center
> >>>> Riesstrasse 25, 80992 München
> >>>>
> >>>> E-mail: radu.tudoran@huawei.com
> >>>> Mobile: +49 15209084330 <+49%201520%209084330>
> >>>> Telephone: +49 891588344173 <+49%2089%201588344173>
> >>>>
> >>>> HUAWEI TECHNOLOGIES Duesseldorf GmbH
> >>>> Hansaallee 205, 40549 Düsseldorf, Germany, www.huawei.com
> >>>> Registered Office: Düsseldorf, Register Court Düsseldorf, HRB 56063,
> >>>> Managing Director: Bo PENG, Wanzhou MENG, Lifang CHEN
> >>>> Sitz der Gesellschaft: Düsseldorf, Amtsgericht Düsseldorf, HRB 56063,
> >>>> Geschäftsführer: Bo PENG, Wanzhou MENG, Lifang CHEN
> >>>> This e-mail and its attachments contain confidential information from
> >>>> HUAWEI, which is intended only for the person or entity whose address
> >> is
> >>>> listed above. Any use of the information contained herein in any way
> >>>> (including, but not limited to, total or partial disclosure,
> >>> reproduction,
> >>>> or dissemination) by persons other than the intended recipient(s) is
> >>>> prohibited. If you receive this e-mail in error, please notify the
> >> sender
> >>>> by phone or email immediately and delete it!
> >>>>
> >>>> -----Original Message-----
> >>>> From: Fabian Hueske [mailto:fhueske@gmail.com]
> >>>> Sent: Tuesday, February 14, 2017 1:01 AM
> >>>> To: dev@flink.apache.org
> >>>> Subject: [DISCUSS] Table API / SQL indicators for event and processing
> >>> time
> >>>> Hi,
> >>>>
> >>>> I'd like to start an discussion about how Table API / SQL queries
> >>> indicate
> >>>> whether an operation is done in event or processing time.
> >>>>
> >>>> 1) Why do we need to indicate the time mode?
> >>>>
> >>>> We need to distinguish event time and processing time mode for
> >> operations
> >>>> in queries in order to have the semantics of a query fully defined.
> >>>> This cannot be globally done in the TableEnvironment because some
> >> queries
> >>>> explicitly request an expression such as the ORDER BY clause of an
> OVER
> >>>> window with PRECEDING / FOLLOWING clauses.
> >>>> So we need a way to specify something like the following query:
> >>>>
> >>>> SELECT
> >>>>    a,
> >>>>    SUM(b) OVER (PARTITION BY c ORDER BY proctime ROWS BETWEEN 2
> >> PRECEDING
> >>>> AND CURRENT ROW) AS sumB, FROM myStream
> >>>>
> >>>> where "proctime" indicates processing time. Equivalently "rowtime"
> >> would
> >>>> indicate event time.
> >>>>
> >>>> 2) Current state
> >>>>
> >>>> The current master branch implements time support only for grouping
> >>>> windows in the Table API.
> >>>> Internally, the Table API converts a 'rowtime symbol (which looks like
> >> a
> >>>> regular attribute) into a special expression which indicates
> >> event-time.
> >>>> For example:
> >>>>
> >>>> table
> >>>>    .window(Tumble over 5.milli on 'rowtime as 'w)
> >>>>    .groupBy('a, 'w)
> >>>>    .select(...)
> >>>>
> >>>> defines a tumbling event-time window.
> >>>>
> >>>> Processing-time is indicated by omitting a time attribute
> >>>> (table.window(Tumble over 5.milli as 'w) ).
> >>>>
> >>>> 3) How can we do that in SQL?
> >>>>
> >>>> In SQL we cannot add special expressions without touching the parser
> >>> which
> >>>> we don't want to do because we want to stick to the SQL standard.
> >>>> Therefore, I see only two options: adding system attributes or
> >>>> (parameterless) built-in functions. I list some pros and cons of the
> >>>> approaches below:
> >>>>
> >>>> 1. System Attributes:
> >>>> + most natural way to access a property of a record.
> >>>> + works with joins, because time attributes can be related to tables
> >>>> - We need to ensure the attributes are not writable and always present
> >> in
> >>>> streaming tables (i.e., they should be system defined attributes).
> >>>> - Need to adapt existing Table API expressions (will not change the
> API
> >>>> but some parts of the internal translation)
> >>>> - Event time value must be set when the stream is converted,
> processing
> >>>> time is evaluated on the fly
> >>>>
> >>>> 2. Built-in Functions
> >>>> + Users could try to modify time attributes which is not possible with
> >>>> functions
> >>>> - do not work with joins, because we need to address different
> >> relations
> >>>> - not a natural way to access a property of a record
> >>>>
> >>>> I think the only viable choice are system attributes, because built-in
> >>>> functions cannot be used for joins.
> >>>> However, system attributes are the more complex solution because they
> >>> need
> >>>> a better integration with Calcite's SQL validator (preventing user
> >>>> attributes which are named rowtime for instance).
> >>>>
> >>>> Since there are currently a several contributions on the way (such as
> >> SQL
> >>>> OVER windows FLINK-5653 to FLINK-5658) that need time indicators, we
> >>> need a
> >>>> solution soon to be able to make progress.
> >>>> There are two PRs, #3252 and #3271, which implement the built-in
> marker
> >>>> functions proctime() and rowtime() and which could serve as a
> temporary
> >>>> solution (since we do not work on joins yet).
> >>>> I would like to suggest to use these functions as a starting point
> >> (once
> >>>> the PRs are merged) and later change to the system attribute solution
> >>> which
> >>>> needs a bit more time to be implemented.
> >>>>
> >>>> I talked with Timo today about this issue and he said he would like
to
> >>>> investigate how we can implement this as system functions properly
> >>>> integrated with Calcite and the SQL Validator.
> >>>>
> >>>> What do others think?
> >>>>
> >>>> Best, Fabian
> >>>>
>
>

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