[Function] Let "str_to_date" return correct type

[morningman]

Proposed changes

The return type of str_to_date depends on whether the time part is included in the format.
If included, it is DATETIME, otherwise it is DATE.
If the format parameter is not constant, the return type will be DATETIME.
The above judgment has been completed in the FE query planning stage,
so here we directly set the value type to the return type set in the query plan.

For example:
A table with one column k1 varchar, and has 2 lines:
"%Y-%m-%d"
"%Y-%m-%d %H:%i:%s"
Query:
SELECT str_to_date("2020-09-01", k1) from tbl;
Result will be:
2020-09-01 00:00:00
2020-09-01 00:00:00

Query:
SELECT str_to_date("2020-09-01", "%Y-%m-%d");
Return type is DATE

Query:
SELECT str_to_date("2020-09-01", "%Y-%m-%d %H:%i:%s");
Return type is DATETIME

Types of changes

• Bugfix (non-breaking change which fixes an issue)

[yangzhg]

+1

[EmmyMiao87]

LGTM