## What changes were proposed in this pull request?
In the PR, I propose new function `date_part()`. The function is modeled on the traditional Ingres equivalent to the SQL-standard function `extract`:
```
date_part('field', source)
```
and added for feature parity with PostgreSQL (https://www.postgresql.org/docs/11/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT).
The `source` can have `DATE` or `TIMESTAMP` type. Supported string values of `'field'` are:
- `millennium` - the current millennium for given date (or a timestamp implicitly casted to a date). For example, years in the 1900s are in the second millennium. The third millennium started _January 1, 2001_.
- `century` - the current millennium for given date (or timestamp). The first century starts at 0001-01-01 AD.
- `decade` - the current decade for given date (or timestamp). Actually, this is the year field divided by 10.
- isoyear` - the ISO 8601 week-numbering year that the date falls in. Each ISO 8601 week-numbering year begins with the Monday of the week containing the 4th of January.
- `year`, `month`, `day`, `hour`, `minute`, `second`
- `week` - the number of the ISO 8601 week-numbering week of the year. By definition, ISO weeks start on Mondays and the first week of a year contains January 4 of that year.
- `quarter` - the quarter of the year (1 - 4)
- `dayofweek` - the day of the week for date/timestamp (1 = Sunday, 2 = Monday, ..., 7 = Saturday)
- `dow` - the day of the week as Sunday (0) to Saturday (6)
- `isodow` - the day of the week as Monday (1) to Sunday (7)
- `doy` - the day of the year (1 - 365/366)
- `milliseconds` - the seconds field including fractional parts multiplied by 1,000.
- `microseconds` - the seconds field including fractional parts multiplied by 1,000,000.
- `epoch` - the number of seconds since 1970-01-01 00:00:00 local time in microsecond precision.
Here are examples:
```sql
spark-sql> select date_part('year', timestamp'2019-08-12 01:00:00.123456');
2019
spark-sql> select date_part('week', timestamp'2019-08-12 01:00:00.123456');
33
spark-sql> select date_part('doy', timestamp'2019-08-12 01:00:00.123456');
224
```
I changed implementation of `extract` to re-use `date_part()` internally.
## How was this patch tested?
Added `date_part.sql` and regenerated results of `extract.sql`.
Closes#25410 from MaxGekk/date_part.
Lead-authored-by: Maxim Gekk <max.gekk@gmail.com>
Co-authored-by: Takeshi Yamamuro <yamamuro@apache.org>
Signed-off-by: Takeshi Yamamuro <yamamuro@apache.org>
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