fengys1996 opened a new issue, #22756:
URL: https://github.com/apache/datafusion/issues/22756
### Describe the bug
When comparing two Timestamp values with different TimeUnits, DataFusion
coerces both sides to the coarser time unit. This may result in a loss of
precision and lead to incorrect comparison semantics.
And other timestamp operations, such as arithmetic operations and
type-unification contexts like `UNION`, also choose the coarser time unit.
### To Reproduce
1. compare ts with different time unit.
```sql
SELECT
CAST(TIMESTAMP '2024-01-01 00:00:00' AS TIMESTAMP(0)) =
CAST(TIMESTAMP '2024-01-01 00:00:00.123' AS TIMESTAMP(3)) AS eq;
```
Expected:
```text
+---------------------------------------------------------------+
| Utf8("2024-01-01 00:00:00") = Utf8("2024-01-01 00:00:00.123") |
+---------------------------------------------------------------+
| false |
+---------------------------------------------------------------+
```
Actual:
```text
+---------------------------------------------------------------+
| Utf8("2024-01-01 00:00:00") = Utf8("2024-01-01 00:00:00.123") |
+---------------------------------------------------------------+
| true |
+---------------------------------------------------------------+
```
2. ts subtraction
```sql
SELECT
CAST(TIMESTAMP '2024-01-01 00:00:00.123' AS TIMESTAMP(3)) -
CAST(TIMESTAMP '2024-01-01 00:00:00' AS TIMESTAMP(0)) AS diff;
```
Expected:
```text
+----------+
| diff |
+----------+
| PT0.123S |
+----------+
```
Actual:
```text
+------+
| diff |
+------+
| P0D |
+------+
```
3. ts union
```sql
SELECT CAST(TIMESTAMP '2024-01-01 00:00:00' AS TIMESTAMP(0))
UNION ALL
SELECT CAST(TIMESTAMP '2024-01-01 00:00:00.123' AS TIMESTAMP(3));
```
Expected:
```text
+-----------------------------+
| Utf8("2024-01-01 00:00:00") |
+-----------------------------+
| 2024-01-01T00:00:00.123 |
| 2024-01-01T00:00:00 |
+-----------------------------+
```
Actual:
```text
+-----------------------------+
| Utf8("2024-01-01 00:00:00") |
+-----------------------------+
| 2024-01-01T00:00:00 |
| 2024-01-01T00:00:00 |
+-----------------------------+
```
### Expected behavior
_No response_
### Additional context
I also ran a few similar queries in ClickHouse. It appears that ClickHouse
prefers the finer precision.
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