hozan23 opened a new issue, #26885:
URL: https://github.com/apache/datafusion/issues/26885
### Describe the bug
`MySqlDialect::timestamp_cast_dtype` always returns `DATETIME` with no
precision. In MySQL, `DATETIME` means `DATETIME(0)`, so a sub-second timestamp
is rounded to whole seconds:
CAST('2026-09-21 08:55:40.006' AS DATETIME) -- 2026-09-21 08:55:40
### To Reproduce
```rust
let unparser = Unparser::new(&MySqlDialect {});
let expr =
Expr::Literal(ScalarValue::TimestampMillisecond(Some(1789980940006), None),
None);
println!("{}", unparser.expr_to_sql(&expr)?);
// CAST('2026-09-21 08:55:40.006' AS DATETIME)
```
### Expected behavior
The cast keeps the time unit's precision: `DATETIME` for seconds,
`DATETIME(3)` for milliseconds, and `DATETIME(6)` for micro- and nanoseconds
(must be in the range 0 to 6).
See https://dev.mysql.com/doc/refman/8.0/en/fractional-seconds.html
### Additional context
_No response_
### Willingness to contribute
I plan to contribute a fix for this bug independently
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