kazuyukitanimura opened a new pull request, #43678:
URL: https://github.com/apache/spark/pull/43678
### What changes were proposed in this pull request?
This PR fixes inaccurate Decimal multiplication and division results.
### Why are the changes needed?
Decimal multiplication and division results may be inaccurate due to
rounding issues.
#### Multiplication:
```
scala> sql("select -14120025096157587712113961295153.858047 *
-0.4652").show(truncate=false)
+----------------------------------------------------+
|(-14120025096157587712113961295153.858047 * -0.4652)|
+----------------------------------------------------+
|6568635674732509803675414794505.574764 |
+----------------------------------------------------+
```
The correct answer is `6568635674732509803675414794505.574763`
Please note that the last digit is `3` instead of `4` as
```
scala>
java.math.BigDecimal("-14120025096157587712113961295153.858047").multiply(java.math.BigDecimal("-0.4652"))
val res21: java.math.BigDecimal = 6568635674732509803675414794505.5747634644
```
Since the factional part `.574763` is followed by `4644`, it should not be
rounded up.
#### Division:
```
scala> sql("select -0.172787979 /
533704665545018957788294905796.5").show(truncate=false)
+-------------------------------------------------+
|(-0.172787979 / 533704665545018957788294905796.5)|
+-------------------------------------------------+
|-3.237521E-31 |
+-------------------------------------------------+
```
The correct answer is `-3.237520E-31`
Please note that the last digit is `0` instead of `1` as
```
scala>
java.math.BigDecimal("-0.172787979").divide(java.math.BigDecimal("533704665545018957788294905796.5"),
100, java.math.RoundingMode.DOWN)
val res22: java.math.BigDecimal =
-3.237520489418037889998826491401059986665344697406144511563561222578738E-31
```
Since the factional part `.237520` is followed by `4894...`, it should not
be rounded up.
### Does this PR introduce _any_ user-facing change?
Yes, users will see correct Decimal multiplication and division results.
Directly multiplying and dividing with
`org.apache.spark.sql.types.Decimal()` (not via SQL) will return 39 digit at
maximum instead of 38 at maximum and round down instead of round half-up
### How was this patch tested?
Test added
### Was this patch authored or co-authored using generative AI tooling?
No
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