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>
