b5eb0933ac
### What changes were proposed in this pull request?
Currently, the legacy fractional formatter is based on the implementation from Spark 2.4 which formats the input timestamp twice:
```
val timestampString = ts.toString
val formatted = legacyFormatter.format(ts)
```
to strip trailing zeros. This PR proposes to avoid the first formatting by forming the second fraction directly.
### Why are the changes needed?
It makes legacy fractional formatter faster.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
By existing test "format fraction of second" in `TimestampFormatterSuite` + added test for timestamps before 1970-01-01 00:00:00Z
Closes #28643 from MaxGekk/optimize-legacy-fract-format.
Authored-by: Max Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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