MaxGekk opened a new pull request #25986: [SPARK-29311][SQL] Return seconds 
with fraction from `date_part()` and `extract`
URL: https://github.com/apache/spark/pull/25986
 
 
   ### What changes were proposed in this pull request?
   
   Added new expression `SecondWithFraction` which produce the `seconds` part 
of timestamps/dates with fractional part containing microseconds. This 
expression is used only in the `DatePart` expression. As the result, 
`date_part()` and `extract` returns seconds and microseconds as the fractional 
part of the seconds part when `field` is `SECOND` (or synonyms).
   
   ### Why are the changes needed?
   
   The `date_part()` and `extract` were added to maintain feature parity with 
PostgreSQL which has different behavior for the `SECOND` value of the `field` 
parameter. The fix is needed to behave in the same way. Here is PostgreSQL's 
output:
   ```sql
   # SELECT date_part('SECONDS', timestamp'2019-10-01 00:00:01.000001');
    date_part 
   -----------
     1.000001
   (1 row)
   ```
   
   ### Does this PR introduce any user-facing change?
   Yes, type of `date_part('SECOND', ...)` is changed from `INT` to `DECIMAL(8, 
6)`.
   Before:
   ```sql
   spark-sql> SELECT date_part('SECONDS', '2019-10-01 00:00:01.000001');
   1
   ```
   After:
   ```sql
   spark-sql> SELECT date_part('SECONDS', '2019-10-01 00:00:01.000001');
   1.000001
   ```
   
   
   ### How was this patch tested?
   - Added new tests to `DateExpressionSuite` for the `SecondWithFraction` 
expression
   - Regenerated results of `date_part.sql`, `extract.sql` and `timestamp.sql`
   - Updated results of `ExtractBenchmark`
   

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