1985437110
### What changes were proposed in this pull request?
```sql
spark-sql> SELECT extract(dayofweek from '2009-07-26');
1
spark-sql> SELECT extract(dow from '2009-07-26');
0
spark-sql> SELECT extract(isodow from '2009-07-26');
7
spark-sql> SELECT dayofweek('2009-07-26');
1
spark-sql> SELECT weekday('2009-07-26');
6
```
Currently, there are 4 types of day-of-week range:
1. the function `dayofweek`(2.3.0) and extracting `dayofweek`(2.4.0) result as of Sunday(1) to Saturday(7)
2. extracting `dow`(3.0.0) results as of Sunday(0) to Saturday(6)
3. extracting` isodow` (3.0.0) results as of Monday(1) to Sunday(7)
4. the function `weekday`(2.4.0) results as of Monday(0) to Sunday(6)
Actually, extracting `dayofweek` and `dow` are both derived from PostgreSQL but have different meanings.
https://issues.apache.org/jira/browse/SPARK-23903
https://issues.apache.org/jira/browse/SPARK-28623
In this PR, we make extracting `dow` as same as extracting `dayofweek` and the `dayofweek` function for historical reason and not breaking anything.
Also, add more documentation to the extracting function to make extract field more clear to understand.
### Why are the changes needed?
Consistency insurance
### Does this PR introduce any user-facing change?
yes, doc updated and extract `dow` is as same as `dayofweek`
### How was this patch tested?
1. modified ut
2. local SQL doc verification
#### before
#### after
Closes #28248 from yaooqinn/SPARK-31474.
Authored-by: Kent Yao <yaooqinn@hotmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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