Re: [DISCUSS] Correct time-related function behavior in Flink SQL

[Kurt Young]

cc this to user & user-zh mailing list because this will affect lots of users, and also quite a lot of users

were asking questions around this topic.

Let me try to understand this from user's perspective.

Your proposal will affect five functions, which are:

• PROCTIME()
• NOW()
• CURRENT_DATE
• CURRENT_TIME
• CURRENT_TIMESTAMP

On Tue, Jan 19, 2021 at 6:42 PM Leonard Xu <[hidden email]> wrote:

I found above example format may mess up in different mail client, I post a picture here[1].

Best,
Leonard

[1] https://github.com/leonardBang/flink-sql-etl/blob/master/etl-job/src/main/resources/pictures/CURRRENT_TIMESTAMP.png <https://github.com/leonardBang/flink-sql-etl/blob/master/etl-job/src/main/resources/pictures/CURRRENT_TIMESTAMP.png>

> 在 2021年1月19日，16:22，Leonard Xu <[hidden email]> 写道：
>
> Hi, all
>
> I want to start the discussion about correcting time-related function behavior in Flink SQL, this is a tricky topic but I think it’s time to address it.
>
> Currently some temporal function behaviors are wired to users.
> 1.  When users use a PROCTIME() in SQL, the value of PROCTIME() has a timezone offset with the wall-clock time in users' local time zone, users need to add their local time zone offset manually to get expected local timestamp(e.g: Users in Germany need to +1h to get expected local timestamp).
>
> 2. Users can not use CURRENT_DATE/CURRENT_TIME/CURRENT_TIMESTAMP  to get wall-clock timestamp in local time zone, and thus they need write UDF in their SQL just for implementing a simple filter like WHERE date_col =  CURRENT_DATE.
>
> 3. Another common case  is the time window  with day interval based on PROCTIME(), user plan to put all data from one day into the same window, but the window is assigned using timestamp in UTC+0 timezone rather than the session timezone which leads to the window starts with an offset(e.g: Users in China need to add -8h in their business sql start and then +8h when output the result, the conversion like a magic for users).
>
> These problems come from that lots of time-related functions like PROCTIME(), NOW(), CURRENT_DATE, CURRENT_TIME and CURRENT_TIMESTAMP are returning time values based on UTC+0 time zone.
>
> This topic will lead to a comparison of the three types, i.e. TIMESTAMP/TIMESTAMP WITHOUT TIME ZONE, TIMESTAMP WITH LOCAL TIME ZONE and TIMESTAMP WITH TIME ZONE. In order to better understand the three types, I wrote a document[1] to help understand them better. You can also know the tree timestamp types behavior in Hadoop ecosystem from the reference link int the doc.
>
>
> I Invested all Flink time-related functions current behavior and compared with other DB vendors like Pg,Presto, Hive, Spark, Snowflake,  I made an excel [2] to organize them well, we can use it for the next discussion. Please let me know if I missed something.
> From my investigation, I think we need to correct the behavior of function NOW()/PROCTIME()/CURRENT_DATE/CURRENT_TIME/CURRENT_TIMESTAMP, to correct them, we can change the function return type or function return value or change return type and return value both. All of those way are valid because SQL:2011 does not specify the function return type and every SQL engine vendor has its own implementation. For example the CURRENT_TIMESTAMP function,
>
> FLINK current behavior        existed problem other vendors' behavior proposed change
> CURRENT_TIMESTAMP     CURRENT_TIMESTAMP
> TIMESTAMP(0) NOT NULL
>
> #session timezone: UTC
> 2020-12-28T23:52:52
>
> #session timezone: UTC+8
> 2020-12-28T23:52:52
>
> wall clock:
> UTC+8: 2020-12-29 07:52:52    Wrong value：returns UTC timestamp, but user expects current timestamp in session time zone      In MySQL, Spark, the function NOW() and CURRENT_TIMESTAMP return current timestamp value in session time zone，the return type is TIMESTAMP
>
> In Pg, Presto, the function NOW() and LOCALTIMESTAMP return current timestamp in session time zone，the return type is TIMESTAMP WITH TIME ZONE
>
> In Snowflake, the function CURRENT_TIMESTAMP / LOCALTIMESTAMP return current timestamp in session time zone，the return type is TIMESTAMP WITH LOCAL TIME ZONE Flink should return current timestamp in session time zone, the return type should be TIMESTAMP
>
>
> I tend to only change the return value for these problematic functions and introduce an option for compatibility consideration, what do you think?
>
>
> Looking forward to your feedback.
>
> Best,
> Leonard
>
> [1] https://docs.google.com/document/d/1iY3eatV8LBjmF0gWh2JYrQR0FlTadsSeuCsksOVp_iA/edit?usp=sharing <https://docs.google.com/document/d/1iY3eatV8LBjmF0gWh2JYrQR0FlTadsSeuCsksOVp_iA/edit?usp=sharing>
> [2] https://docs.google.com/spreadsheets/d/1T178krh9xG-WbVpN7mRVJ8bzFnaSJx3l-eg1EWZe_X4/edit?usp=sharing <https://docs.google.com/spreadsheets/d/1T178krh9xG-WbVpN7mRVJ8bzFnaSJx3l-eg1EWZe_X4/edit?usp=sharing>

[Jark Wu-3]

Great examples to understand the problem and the proposed changes, @Kurt!

Thanks Leonard for investigating this problem.
The time-zone problems around time functions and windows have bothered a lot of users. It's time to fix them!

The return value changes sound reasonable to me, and keeping the return type unchanged will minimize the surprise to the users.
Besides that, I think it would be better to mention how this affects the window behaviors, and the interoperability with DataStream. 

I think this definitely deserves a FLIP.

====================================================

Hi zhisheng,

Do you have examples to illustrate which case will get the wrong window boundaries? 

That will help to verify whether the proposed changes can solve your problem. 

Best,

Jark

On Thu, 21 Jan 2021 at 12:54, zhisheng <[hidden email]> wrote:

Thanks to Leonard Xu for discussing this tricky topic. At present, there are many Flink jobs in our production environment that are used to count day-level reports (eg: count PV/UV ).&nbsp;


If use the default Flink SQL,&nbsp; the window time range of the statistics is incorrect, then the statistical results will naturally be incorrect.&nbsp;


The user needs to deal with the time zone manually in order to solve the problem.&nbsp;


If Flink itself can solve these time zone issues, then I think it will be user-friendly.


Thank you


Best!&nbsp;
zhisheng


------------------&nbsp;原始邮件&nbsp;------------------
发件人:                                                                                                                        "dev"                                                                                    <[hidden email]&gt;;
发送时间:&nbsp;2021年1月19日(星期二) 晚上6:35
收件人:&nbsp;"dev"<[hidden email]&gt;;

主题:&nbsp;Re: [DISCUSS] Correct time-related function behavior in Flink SQL



I found above example format may mess up in different mail client, I post a picture here[1].

Best,
Leonard

[1] https://github.com/leonardBang/flink-sql-etl/blob/master/etl-job/src/main/resources/pictures/CURRRENT_TIMESTAMP.png <https://github.com/leonardBang/flink-sql-etl/blob/master/etl-job/src/main/resources/pictures/CURRRENT_TIMESTAMP.png&gt;

&gt; 在 2021年1月19日，16:22，Leonard Xu <[hidden email]&gt; 写道：
&gt;
&gt; Hi, all
&gt;
&gt; I want to start the discussion about correcting time-related function behavior in Flink SQL, this is a tricky topic but I think it’s time to address it.
&gt;
&gt; Currently some temporal function behaviors are wired to users.
&gt; 1.&nbsp; When users use a PROCTIME() in SQL, the value of PROCTIME() has a timezone offset with the wall-clock time in users' local time zone, users need to add their local time zone offset manually to get expected local timestamp(e.g: Users in Germany need to +1h to get expected local timestamp).
&gt;
&gt; 2. Users can not use CURRENT_DATE/CURRENT_TIME/CURRENT_TIMESTAMP&nbsp; to get wall-clock timestamp in local time zone, and thus they need write UDF in their SQL just for implementing a simple filter like WHERE date_col =&nbsp; CURRENT_DATE.
&gt;
&gt; 3. Another common case&nbsp; is the time window&nbsp; with day interval based on PROCTIME(), user plan to put all data from one day into the same window, but the window is assigned using timestamp in UTC+0 timezone rather than the session timezone which leads to the window starts with an offset(e.g: Users in China need to add -8h in their business sql start and then +8h when output the result, the conversion like a magic for users).
&gt;
&gt; These problems come from that lots of time-related functions like PROCTIME(), NOW(), CURRENT_DATE, CURRENT_TIME and CURRENT_TIMESTAMP are returning time values based on UTC+0 time zone.
&gt;
&gt; This topic will lead to a comparison of the three types, i.e. TIMESTAMP/TIMESTAMP WITHOUT TIME ZONE, TIMESTAMP WITH LOCAL TIME ZONE and TIMESTAMP WITH TIME ZONE. In order to better understand the three types, I wrote a document[1] to help understand them better. You can also know the tree timestamp types behavior in Hadoop ecosystem from the reference link int the doc.
&gt;
&gt;
&gt; I Invested all Flink time-related functions current behavior and compared with other DB vendors like Pg,Presto, Hive, Spark, Snowflake,&nbsp; I made an excel [2] to organize them well, we can use it for the next discussion. Please let me know if I missed something.
&gt; From my investigation, I think we need to correct the behavior of function NOW()/PROCTIME()/CURRENT_DATE/CURRENT_TIME/CURRENT_TIMESTAMP, to correct them, we can change the function return type or function return value or change return type and return value both. All of those way are valid because SQL:2011 does not specify the function return type and every SQL engine vendor has its own implementation. For example the CURRENT_TIMESTAMP function,
&gt;
&gt; FLINK      current behavior        existed problem other vendors' behavior proposed change
&gt; CURRENT_TIMESTAMP  CURRENT_TIMESTAMP
&gt; TIMESTAMP(0) NOT NULL
&gt;
&gt; #session timezone: UTC
&gt; 2020-12-28T23:52:52
&gt;
&gt; #session timezone: UTC+8
&gt; 2020-12-28T23:52:52
&gt;
&gt; wall clock:
&gt; UTC+8: 2020-12-29 07:52:52 Wrong value：returns UTC timestamp, but user expects current timestamp in session time zone      In MySQL, Spark, the function NOW() and CURRENT_TIMESTAMP return current timestamp value in session time zone，the return type is TIMESTAMP
&gt;
&gt; In Pg, Presto, the function NOW() and LOCALTIMESTAMP return current timestamp in session time zone，the return type is TIMESTAMP WITH TIME ZONE
&gt;
&gt; In Snowflake, the function CURRENT_TIMESTAMP / LOCALTIMESTAMP return current timestamp in session time zone，the return type is TIMESTAMP WITH LOCAL TIME ZONE      Flink should return current timestamp in session time zone, the return type should be TIMESTAMP
&gt;
&gt;
&gt; I tend to only change the return value for these problematic functions and introduce an option for compatibility consideration, what do you think?
&gt;
&gt;
&gt; Looking forward to your feedback.
&gt;
&gt; Best,
&gt; Leonard
&gt;
&gt; [1] https://docs.google.com/document/d/1iY3eatV8LBjmF0gWh2JYrQR0FlTadsSeuCsksOVp_iA/edit?usp=sharing <https://docs.google.com/document/d/1iY3eatV8LBjmF0gWh2JYrQR0FlTadsSeuCsksOVp_iA/edit?usp=sharing&gt;
&gt; [2] https://docs.google.com/spreadsheets/d/1T178krh9xG-WbVpN7mRVJ8bzFnaSJx3l-eg1EWZe_X4/edit?usp=sharing <https://docs.google.com/spreadsheets/d/1T178krh9xG-WbVpN7mRVJ8bzFnaSJx3l-eg1EWZe_X4/edit?usp=sharing&gt;

[Leonard Xu]

To Kurt, thanks  for the intuitive case, it really clear, you’re wright that I want to propose to change the return value of these functions. It’s the most important part of the topic from user's perspective.

I think this definitely deserves a FLIP.

To Jark,  nice suggestion, I prepared a FLIP for this topic, and will start the FLIP discussion soon.

If use the default Flink SQL,&nbsp; the window time range of the
statistics is incorrect, then the statistical results will naturally be
incorrect.

To zhisheng, sorry to hear that this problem influenced your production jobs,  Could you share your SQL pattern?  we can have more inputs and try to resolve them.

Best,

Leonard

On Tue, Jan 19, 2021 at 6:42 PM Leonard Xu <[hidden email]> wrote:

I found above example format may mess up in different mail client, I post a picture here[1].

Best,
Leonard

[1] https://github.com/leonardBang/flink-sql-etl/blob/master/etl-job/src/main/resources/pictures/CURRRENT_TIMESTAMP.png <https://github.com/leonardBang/flink-sql-etl/blob/master/etl-job/src/main/resources/pictures/CURRRENT_TIMESTAMP.png>

> 在 2021年1月19日，16:22，Leonard Xu <[hidden email]> 写道：
>
> Hi, all
>
> I want to start the discussion about correcting time-related function behavior in Flink SQL, this is a tricky topic but I think it’s time to address it.
>
> Currently some temporal function behaviors are wired to users.
> 1.  When users use a PROCTIME() in SQL, the value of PROCTIME() has a timezone offset with the wall-clock time in users' local time zone, users need to add their local time zone offset manually to get expected local timestamp(e.g: Users in Germany need to +1h to get expected local timestamp).
>
> 2. Users can not use CURRENT_DATE/CURRENT_TIME/CURRENT_TIMESTAMP  to get wall-clock timestamp in local time zone, and thus they need write UDF in their SQL just for implementing a simple filter like WHERE date_col =  CURRENT_DATE.
>
> 3. Another common case  is the time window  with day interval based on PROCTIME(), user plan to put all data from one day into the same window, but the window is assigned using timestamp in UTC+0 timezone rather than the session timezone which leads to the window starts with an offset(e.g: Users in China need to add -8h in their business sql start and then +8h when output the result, the conversion like a magic for users).
>
> These problems come from that lots of time-related functions like PROCTIME(), NOW(), CURRENT_DATE, CURRENT_TIME and CURRENT_TIMESTAMP are returning time values based on UTC+0 time zone.
>
> This topic will lead to a comparison of the three types, i.e. TIMESTAMP/TIMESTAMP WITHOUT TIME ZONE, TIMESTAMP WITH LOCAL TIME ZONE and TIMESTAMP WITH TIME ZONE. In order to better understand the three types, I wrote a document[1] to help understand them better. You can also know the tree timestamp types behavior in Hadoop ecosystem from the reference link int the doc.
>
>
> I Invested all Flink time-related functions current behavior and compared with other DB vendors like Pg,Presto, Hive, Spark, Snowflake,  I made an excel [2] to organize them well, we can use it for the next discussion. Please let me know if I missed something.
> From my investigation, I think we need to correct the behavior of function NOW()/PROCTIME()/CURRENT_DATE/CURRENT_TIME/CURRENT_TIMESTAMP, to correct them, we can change the function return type or function return value or change return type and return value both. All of those way are valid because SQL:2011 does not specify the function return type and every SQL engine vendor has its own implementation. For example the CURRENT_TIMESTAMP function,
>
> FLINK current behavior        existed problem other vendors' behavior proposed change
> CURRENT_TIMESTAMP     CURRENT_TIMESTAMP
> TIMESTAMP(0) NOT NULL
>
> #session timezone: UTC
> 2020-12-28T23:52:52
>
> #session timezone: UTC+8
> 2020-12-28T23:52:52
>
> wall clock:
> UTC+8: 2020-12-29 07:52:52    Wrong value：returns UTC timestamp, but user expects current timestamp in session time zone      In MySQL, Spark, the function NOW() and CURRENT_TIMESTAMP return current timestamp value in session time zone，the return type is TIMESTAMP
>
> In Pg, Presto, the function NOW() and LOCALTIMESTAMP return current timestamp in session time zone，the return type is TIMESTAMP WITH TIME ZONE
>
> In Snowflake, the function CURRENT_TIMESTAMP / LOCALTIMESTAMP return current timestamp in session time zone，the return type is TIMESTAMP WITH LOCAL TIME ZONE Flink should return current timestamp in session time zone, the return type should be TIMESTAMP
>
>
> I tend to only change the return value for these problematic functions and introduce an option for compatibility consideration, what do you think?
>
>
> Looking forward to your feedback.
>
> Best,
> Leonard
>
> [1] https://docs.google.com/document/d/1iY3eatV8LBjmF0gWh2JYrQR0FlTadsSeuCsksOVp_iA/edit?usp=sharing <https://docs.google.com/document/d/1iY3eatV8LBjmF0gWh2JYrQR0FlTadsSeuCsksOVp_iA/edit?usp=sharing>
> [2] https://docs.google.com/spreadsheets/d/1T178krh9xG-WbVpN7mRVJ8bzFnaSJx3l-eg1EWZe_X4/edit?usp=sharing <https://docs.google.com/spreadsheets/d/1T178krh9xG-WbVpN7mRVJ8bzFnaSJx3l-eg1EWZe_X4/edit?usp=sharing>