presto-iceberg/src/main/java/io/prestosql/plugin/iceberg/PartitionTransforms.java/PartitionTransforms/truncateDecimal
class PartitionTransforms:    private static BigDecimal truncateDecimal(BigDecimal value, BigInteger unscaledWidth)
    {
        BigDecimal remainder = new BigDecimal(
                value.unscaledValue()
                        .remainder(unscaledWidth)
                        .add(unscaledWidth)
                        .remainder(unscaledWidth),
                value.scale());
        return value.subtract(remainder);
    }