Hi Leonard,
thanks for working on this topic. I agree that time handling is not easy
in Flink at the moment. We added new time data types (and some are still
not supported which even further complicates things like TIME(9)). We
should definitely improve this situation for users.
This is a pretty opinionated topic and it seems that the SQL standard is
not really deciding this but is at least supporting. So let me express
my opinion for the most important functions:
LOCALDATE / LOCALTIME / LOCALTIMESTAMP
--> uses session time zone, returns DATE/TIME/TIMESTAMP
I think those are the most obvious ones because the LOCAL indicates that
the locality should be materialized into the result and any time zone
information (coming from session config or data) is not important
afterwards.
CURRENT_DATE/CURRENT_TIME/CURRENT_TIMESTAMP
--> uses session time zone, returns DATE/TIME/TIMESTAMP
I'm very sceptical about this behavior. Almost all mature systems
(Oracle, Postgres) and new high quality systems (Presto, Snowflake) use
a data type with some degree of time zone information encoded. In a
globalized world with businesses spanning different regions, I think we
should do this as well. There should be a difference between
CURRENT_TIMESTAMP and LOCALTIMESTAMP. And users should be able to choose
which behavior they prefer for their pipeline.
If we would design this from scatch, I would suggest the following:
- drop CURRENT_DATE / CURRENT_TIME and let users pick LOCALDATE /
LOCALTIME for materialized timestamp parts
- CURRENT_TIMESTAMP should return a TIMESTAMP WITH TIME ZONE to
materialize all session time information into every record. It it the
most generic data type and allows to cast to all other timestamp data
types. This generic ability can be used for filter predicates as well
either through implicit or explicit casting.
PROCTIME/ROWTIME should be time functions based on a long value. Both
System.currentMillis() and our watermark system work on long values.
Those should return TIMESTAMP WITH LOCAL TIME ZONE because the main
calculation should always happen based on UTC. We discussed it in a
different thread, but we should allow PROCTIME globally. People need a
way to create instances of TIMESTAMP WITH LOCAL TIME ZONE. This is not
considered in the current design doc. Many pipelines contain UTC
timestamps and thus it should be easy to create one. Also, both
CURRENT_TIMESTAMP and LOCALTIMESTAMP can work with this type because we
should remember that TIMESTAMP WITH LOCAL TIME ZONE accepts all
timestamp data types as casting target [1]. We could allow TIMESTAMP
WITH TIME ZONE in the future for ROWTIME.
In any case, windows should simply adapt their behavior to the passed
timestamp type. And with TIMESTAMP WITH LOCAL TIME ZONE a day is defined
by considering the current session time zone.
If we would like to design this with less effort required, we could
think about returning TIMESTAMP WITH LOCAL TIME ZONE also for
CURRENT_TIMESTAMP.
I will try to involve more people into this discussion.
Thanks,
Timo
[1]
https://docs.oracle.com/en/database/oracle/oracle-database/21/sqlrf/Data-Types.html#GUID-E7CA339A-2093-4FE4-A36E-1D09593591D3
On 21.01.21 15:32, Leonard Xu wrote:
Before the changes, as I am writing this reply, the local time here is
2021-01-21 12:03:35 (Beijing time, UTC+8).
And I tried these 5 functions in sql client, and got:
Flink SQL> select now(), PROCTIME(), CURRENT_TIMESTAMP, CURRENT_DATE,
CURRENT_TIME;
+-------------------------+-------------------------+-------------------------+--------------+--------------+
| EXPR$0 | EXPR$1 | CURRENT_TIMESTAMP |
CURRENT_DATE | CURRENT_TIME |
+-------------------------+-------------------------+-------------------------+--------------+--------------+
| 2021-01-21T04:03:35.228 | 2021-01-21T04:03:35.228 | 2021-01-21T04:03:35.228 |
2021-01-21 | 04:03:35.228 |
+-------------------------+-------------------------+-------------------------+--------------+--------------+
After the changes, the expected behavior will change to:
Flink SQL> select now(), PROCTIME(), CURRENT_TIMESTAMP, CURRENT_DATE,
CURRENT_TIME;
+-------------------------+-------------------------+-------------------------+--------------+--------------+
| EXPR$0 | EXPR$1 | CURRENT_TIMESTAMP |
CURRENT_DATE | CURRENT_TIME |
+-------------------------+-------------------------+-------------------------+--------------+--------------+
| 2021-01-21T12:03:35.228 | 2021-01-21T12:03:35.228 | 2021-01-21T12:03:35.228 |
2021-01-21 | 12:03:35.228 |
+-------------------------+-------------------------+-------------------------+--------------+--------------+
The return type of now(), proctime() and CURRENT_TIMESTAMP still be TIMESTAMP;
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, 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 <xbjt...@gmail.com
<mailto:xbjt...@gmail.com>> 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>
<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 <xbjt...@gmail.com <mailto:xbjt...@gmail.com>> 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>
<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>
<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>>
