Status：

Current state: under discussion

Discussion thread: http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/DISCUSS-FLIP-162-Consistent-Flink-SQL-time-function-behavior-td40883.html

JIRA: 

Released: 1.13.0

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

Currently some temporal function behaviors are weird to users. 

• When users use a PROCTIME() in SQL, the return 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). 
• 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. 

• 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 the fact 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 FLIP aims to consistent the timestamp function behavior and eventually improve the usability.

Public Interfaces 

As we knew some functions' behavior is wrong currently, but after we correct these function, the legacy behavior should still work in old code.

This FLIP introduce an option for compatibility consideration, given two option names, I prefer the first one, the second one is like Impala style[1].

table.exec.fallback-legacy-time-function = false/true 

table.exec.use-utc-for-unixtime-conversion = false/true    

Proposed Changes

1. Correct the return value of time functions

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.

To correct our current behavior, we need to make the function return type clear, especially for timestamp type, i.e.

• TIMESTAMP/TIMESTAMP WITHOUT TIME ZONE
• TIMESTAMP WITH LOCAL TIME ZONE
• TIMESTAMP WITH TIME ZONE

In order to understand these types better, I wrote a document[3]. BTW, Flink also keeps same semantics for three timestamp types comparing with Hadoop ecosystem.

From my investigation, to correct this time functions' behavior, we have several options
(1) change the function return type
(2) change the function return value
(3) change them 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[2], for example CURRENT_TIMESTAMP.

NOTE: Flink supports TIME-related types with precision well,  all example in this FLIP the precision just retains to seconds for simplification purpose.

This FLIP proposes option 1 which changes the return type, the proposed changes as following, the current wall-clock is 2020-12-29 07:52:52 in Beijing time(UTC+8):

After the proposal, the function NOW() and CURRENT_TIMESTAMP  become  synonyms, the function CURRENT_TIME and LOCALTIME become synonyms, you can also lookup all time function behaviors in reference [2]. 

2. Disable CAST between NUMERIC  and TIMESTAMP

Currently, the following CAST conversion behaviors are wrong which does not consider the session time zone. It should use session time zone when cast between NUMERIC type and TIMESTAMP type if there’re strong requirements to support this , the numeric type include TINYINT, SMALLINT, INT, BIGINT, FLOAT, DOUBLE, they keep same conversion behavior, for example the cast between BIGINT and TIMESTAMP. Although we didn’t expose this feature in the document, some users may use them.

These cast conversions have wrong behavior and problematic semantics,  because SQL:2011 does not contains these cast specification and we never expose to user.

This FLIP propose to disable the cast between numeric and timestamp,  and introduce a the function TO_TIMESTAMP(seconds) function which accepts a BIGINT seconds and return a TIMESTAMP in local time zone. 

3. Introduce an option that enable fallback to legacy behavior

 The default value of table.exec.fallback-legacy-time-function is 'false' which means: 

        (1) the return values of function

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

       consider the local timezone and the return type of PROCTIME()/NOW()/CURRENT_TIMESTAMP is TIMESTAMP WITH LOCAL TIME ZONE, the return type of CURRENT_DATE is DATE, the return type of CURRENT_TIME is TIME.

      (2) The cast between NUMERIC and TIMESTAMP is forbidden

Users can set the option to 'true', at this moment:

      (1) These functions would keep the legacy behavior.

      (2) The cast between NUMERIC and TIMESTAMP is supported.

4. Support defining row time attribute on TIMESTAMP WITH LOCAL TIME ZONE

After this, we can support ROWTIME/PROCTIME on type TIMESTAMP WITH LOCAL TIME ZONE, which complements the unifcation.

For usability consideration, this FLIP also propose to introduce a function TO_TIMETSTAMP_LTZ(numeric [,]) which is similar to above TO_TIMESTAMP function but the returns type is TIMESTAMP WITH LOCAL TIME ZONE.

5.  Support more conversion classes for LocalZonedTimestampType(TIMESTAMP WITH LOCAL TIME ZONE)

After we use  type TIMESTAMP WITH LOCAL TIME ZONE in above functions, we can support all  conversion classes like java.time.LocalDateTime, java.sql.Timestamp that TimestampType supported  for LocalZonedTimestampType to resolve the UDF compatibility issue.

For example, if user used a UDF which parameter types contains LocalDateTime, and the SQL column data type is TIMESTAMP comes from PROCTIME()/CURRENT_TIMESTAMP, they can migrate to TIMESTAMP WITH LOCAL TIME ZONE smoothly.

6. Support abbreviation name for all timestamp types 

The type name of timestamp is pretty long and inconvenient for users, we can introduce abbreviation for them to make it more convenient.

• TIMESTAMP WITHOUT TIME ZONE               <=> TIMESTAMP_NTZ   which also equals TIMESTAMP
• TIMESTAMP WITH LOCAL TIME ZONE          <=> TIMESTAMP_LTZ  
• TIMESTAMP_WITH TIME ZONE                      <=> TIMESTAMP_TZ 

7. Support TIME(9) for Flink SQL

Due to the historical reason, we didn't not support TIME(9) yet, we think It's a good time point to support it in this FLIP.

General Implementations

1.Change the codegen implementations for above five functions/cast conversions according to the value of introduced table option: table.exec.fallback-legacy-time-function

2. Supports all  conversion classes like java.time.LocalDateTime, java.sql.Timestamp that TimestampType supported  for LocalZonedTimestampType to resolve the UDF compatibility issue

3. The session timezone offset for processing-time window should still be considered

4. All connectors/formats should supports TIMESTAMP WITH LOCAL TIME ZONE well 

5. we also should record in document  

After the proposal is finished, the above user cases will work smoothly.  Assume users' local time zone is UTC+8, the wall-clock is 2020-12-29 07:52:52.

• user case 1 :

Flink SQL> SELECT NOW(), PROCTIME(), CURRENT_TIMESTAMP, CURRENT_DATE, CURRENT_TIME;
-- output:
+-------------------------+-------------------------+-------------------------+--------------+--------------+
|                   NOW() |              PROCTIME() |       CURRENT_TIMESTAMP | CURRENT_DATE | CURRENT_TIME |
+-------------------------+-------------------------+-------------------------+--------------+--------------+
|     2020-12-29 07:52:52 |     2020-12-29 07:52:52 |     2020-12-29 07:52:52 |   2020-12-29 |      07:52:52|
+-------------------------+-------------------------+-------------------------+--------------+--------------+

• user case 2: 

Flink SQL> SELECT TUMBLE_START(proctime, INTERVAL ‘1’ DAY),
         >        TUMBLE_END(proctime, INTERVAL ‘1’ DAY),
         >        count(userId) as cnt
         >    FROM userLog
         >    GROUP BY TUMBLE_WINDOW(proctime, INTERVAL ‘1’ DAY);

-- output:
+-------------------------+-------------------------+-------------------------+ 
|            TUMBLE_START |              TUMBLE_END |           count(userId) | 
+-------------------------+-------------------------+-------------------------+ 
|     2020-12-29 00:00:00 |     2020-12-30 00:00:00 |     		  		  100 |
+-------------------------+-------------------------+-------------------------+ 

• user case 3:

Flink SQL> SELECT  *
         >    FROM userLog
         >    WHERE date_col >= CURRENT_DATE;

-- in the query, records earlier than 2020-12-29 will not be output.
+-------------------------+-------------------------+-------------------------+ 
|                date_col |                  log_ts |                    user | 
+-------------------------+-------------------------+-------------------------+ 
|             2020-12-29  |     2020-12-29 00:00:00 |     		  		Alice |
+-------------------------+-------------------------+-------------------------+ 
|             2020-12-29  |     2020-12-29 00:00:01 |     		  		  Bob |
+-------------------------+-------------------------+-------------------------+ 
|             2020-12-29  |     2020-12-29 00:00:02 |     		  		  Tom |
+-------------------------+-------------------------+-------------------------+ 

Compatibility, Deprecation, and Migration Plan

• Compatibility

This is an incompatible change, we introduce SQL/API option  table.exec.fallback-legacy-time-function for compacting current wrong behavior, and set it to ‘false’. If users want to keep the legacy behavior, they need to set this option value to ’true’ manually, this would be add to release note. 

Test Plan

Will add plan tests, unit tests, window operator harness tests as well as IT tests. 

Rejected Alternatives

1. Change the return type of function CURRENT_TIMESTAMP/NOW()/PROCETIME() to TIMESTAMP WITH TIME ZONE. This proposal needs to introduce a new type TIMESTAMP WITH TIME ZONE, and we think there are no enough benefits. If we do this, the return type of function CURRENT_TIME must be TIME WITH TIME ZONE for consistent consideration, we need to introduce another type.

2. Change the return value of function  CURRENT_TIMESTAMP/NOW()/PROCETIME() to the timestamp value in local timezone. The proposal is fine if we only use it in FLINK SQL world, but we need to consider the conversion between Table/DataStream, assume a record produced in UTC+0 timezone with TIMESTAMP '1970-01-01 08:00:44'  and the Flink SQL processes the data with session time zone 'UTC+8', if the sql program need to convert the Table to DataStream, then we need to calculate the timestamp in StreamRecord with session time zone (UTC+8), then we will get 44 in DataStream program, but it is wrong because the expected value should be (8 * 60 * 60 + 44). The corner case tell us that the ROWTIME/PROCTIME in Flink are based on UTC+0, when correct the PROCTIME() function, the better way is to use TIMESTAMP WITH LOCAL TIME ZONE which keeps same long value with time based on UTC+0 and can be expressed with  local timezone.

References

1. https://docs.cloudera.com/documentation/enterprise/latest/topics/impala_timezone.html

      2. https://docs.google.com/document/d/1iY3eatV8LBjmF0gWh2JYrQR0FlTadsSeuCsksOVp_iA/edit?usp=sharing

      3. https://docs.google.com/spreadsheets/d/1T178krh9xG-WbVpN7mRVJ8bzFnaSJx3l-eg1EWZe_X4/edit?usp=sharing