[GitHub] [spark] MaxGekk opened a new pull request #24332: [SPARK-27423][SQL] Cast DATE <-> TIMESTAMP according to the SQL standard

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MaxGekk opened a new pull request #24332: [SPARK-27423][SQL] Cast DATE <-> TIMESTAMP according to the SQL standard
URL: https://github.com/apache/spark/pull/24332
 
 
   ## What changes were proposed in this pull request?
   
   According to SQL standard, value of `DATE` type is union of year, month, dayInMonth, and it is independent from any time zones. To convert it to Catalyst's `TIMESTAMP` which is actually `TIMESTAMP WITH TIME ZONE`, DATE value should be "extended" by the time at midnight - `00:00:00`. The resulted local date+time should be considered as a timestamp in the session time zone, and casted to microseconds since epoch in `UTC` accordingly. 
   
   The reverse casting from `TIMESTAMP` to `DATE` should be performed in the similar way. `TIMESTAMP` values should be represented as a local date+time in the session time zone. And the time component should be just removed. For example, `TIMESTAMP 2019-04-10 00:10:12` -> `DATE 2019-04-10`. The resulted date is converted to days since epoch `1970-01-01`. 
   
   ## How was this patch tested?
   
   The changes were tested by existing test suites - `DateFunctionsSuite`, `DateExpressionsSuite` and `CastSuite`.

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