yaooqinn opened a new pull request #28674:
URL: https://github.com/apache/spark/pull/28674
### What changes were proposed in this pull request?
In 3.0, the new approach in datetime parser can not extract the
week-based-year field, so it goes
idempotently to 1970.
```sql
spark-sql> explain select to_timestamp('1969-01-01', 'YYYY-MM-dd');
== Physical Plan ==
*(1) Project [-28800000000 AS to_timestamp(1969-01-01, YYYY-MM-dd)#37]
+- *(1) Scan OneRowRelation[]
== Physical Plan ==
*(1) Project [-28800000000 AS to_timestamp(2000-01-01, YYYY-MM-dd)#73]
+- *(1) Scan OneRowRelation[]
```
In Legacy mode a.k.a version 2.4, the result will be the last Sunday of the
past year(It is weird too!)
```sql
spark-sql> select to_timestamp('1969-01-01', 'YYYY-MM-dd');
1968-12-29 00:00:00
spark-sql> select to_timestamp('1969-01-02', 'YYYY-MM-dd');
1968-12-29 00:00:00
spark-sql> select to_timestamp('1969-01-03', 'YYYY-MM-dd');
1968-12-29 00:00:00
spark-sql> select to_timestamp('1969-01-04', 'YYYY-MM-dd');
1968-12-29 00:00:00
spark-sql> select to_timestamp('1969-01-05', 'YYYY-MM-dd');
1968-12-29 00:00:00
spark-sql> select to_timestamp('1969-01-06', 'YYYY-MM-dd');
1968-12-29 00:00:00
spark-sql> select to_timestamp('1969-01-07', 'YYYY-MM-dd');
1968-12-29 00:00:00
spark-sql> select to_timestamp('1969-01-31', 'YYYY-MM-dd');
1968-12-29 00:00:00
spark-sql> select to_timestamp('1969-02-28', 'YYYY-MM-dd');
1968-12-29 00:00:00
spark-sql> select to_timestamp('1969-12-28', 'YYYY-MM-dd');
1968-12-29 00:00:00
spark-sql> select to_timestamp('1969-12-31', 'YYYY-MM-dd');
1968-12-29 00:00:00
```
In this PR, I propose to restore the behavior in 2.4 before we have final
conclusion and do not block releasing 3.0. Since in 3.0 it is much weirder.
FYI, Postgres may set a good example here,
https://www.postgresql.org/docs/9.0/functions-formatting.html
> Caution
--
While to_date will reject a mixture of Gregorian and ISO week-numbering date
fields, to_char will not, since output format specifications like YYYY-MM-DD
(IYYY-IDDD) can be useful. But avoid writing something like IYYY-MM-DD; that
would yield surprising results near the start of the year.
### Why are the changes needed?
bugfix and behavior change restoration
### Does this PR introduce _any_ user-facing change?
No, the behavior will be restored.
### How was this patch tested?
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added new tests
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