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fix: grouping separator for float and decimal #20268

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Merged
kosiew merged 3 commits into from
Apr 30, 2026
+401 −16
Merged

fix: grouping separator for float and decimal #20268

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cf8997e
fix: grouping separator for float and decimal in format_string
Druva-D Apr 27, 2026
8036182
Address review comments
Druva-D Apr 28, 2026
fffdf4e
Add test to document behaviour mismatch
Druva-D Apr 30, 2026
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296 changes: 293 additions & 3 deletions datafusion/spark/src/function/string/format_string.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1690,6 +1690,23 @@ impl ConversionSpecifier {
}

fn format_float(&self, writer: &mut String, value: f64) -> Result<()> {
// Java/Spark reject the grouping separator flag with scientific notation
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if self.grouping_separator
&& matches!(
self.conversion_type,
ConversionType::SciFloatLower | ConversionType::SciFloatUpper
)
{
return exec_err!(
"Grouping separator ',' flag is not compatible with scientific notation conversion '{}'",
if self.conversion_type == ConversionType::SciFloatUpper {
'E'
} else {
'e'
}
);
}

let mut prefix = String::new();
let mut suffix = String::new();
let mut number = String::new();
Expand Down Expand Up @@ -1770,6 +1787,9 @@ impl ConversionSpecifier {
if strip_trailing_0s {
number = trim_trailing_0s(&number).to_owned();
}
if self.grouping_separator {
number = insert_thousands_separator(&number);
}
}
if self.alt_form && !number.contains('.') {
number += ".";
Expand Down Expand Up @@ -2000,6 +2020,23 @@ impl ConversionSpecifier {
}

fn format_decimal(&self, writer: &mut String, value: &str, scale: i64) -> Result<()> {
// Java/Spark reject the grouping separator flag with scientific notation
if self.grouping_separator
&& matches!(
self.conversion_type,
ConversionType::SciFloatLower | ConversionType::SciFloatUpper
)
{
return exec_err!(
"Grouping separator ',' flag is not compatible with scientific notation conversion '{}'",
if self.conversion_type == ConversionType::SciFloatUpper {
'E'
} else {
'e'
}
);
}

let mut prefix = String::new();
let upper = self.conversion_type.is_upper();

Expand Down Expand Up @@ -2033,7 +2070,15 @@ impl ConversionSpecifier {
let number = match self.conversion_type {
ConversionType::DecFloatLower => {
// Format as fixed-point decimal
self.format_decimal_fixed(&abs_decimal, precision, strip_trailing_0s)?
let mut n = self.format_decimal_fixed(
&abs_decimal,
precision,
strip_trailing_0s,
)?;
if self.grouping_separator {
n = insert_thousands_separator(&n);
}
n
}
ConversionType::SciFloatLower => self.format_decimal_scientific(
&abs_decimal,
Expand Down Expand Up @@ -2062,11 +2107,15 @@ impl ConversionSpecifier {
strip_trailing_0s,
)?
} else {
self.format_decimal_fixed(
let mut n = self.format_decimal_fixed(
&abs_decimal,
precision - 1 - log10_val.floor() as i32,
strip_trailing_0s,
)?
)?;
if self.grouping_separator {
n = insert_thousands_separator(&n);
}
n
}
}
_ => {
Expand Down Expand Up @@ -2332,6 +2381,24 @@ impl FloatBits for f64 {
}
}

/// Inserts thousands separators (`,`) into the integer part of a numeric string.
/// For example, `"1234567.89"` becomes `"1,234,567.89"`.
fn insert_thousands_separator(number: &str) -> String {
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let (int_part, frac_part) = match number.find('.') {
Some(pos) => (&number[..pos], &number[pos..]),
None => (number, ""),
};
let mut result = String::with_capacity(number.len() + number.len() / 3);
for (i, c) in int_part.char_indices() {
if i > 0 && (int_part.len() - i) % 3 == 0 {
result.push(',');
}
result.push(c);
}
result.push_str(frac_part);
result
}

fn trim_trailing_0s(number: &str) -> &str {
if number.contains('.') {
for (i, c) in number.chars().rev().enumerate() {
Expand All @@ -2355,6 +2422,8 @@ fn trim_trailing_0s_hex(number: &str) -> &str {
#[cfg(test)]
mod tests {
use super::*;
use crate::function::utils::test::test_scalar_function;
use arrow::array::StringArray;
use arrow::datatypes::DataType::Utf8;

#[test]
Expand Down Expand Up @@ -2385,4 +2454,225 @@ mod tests {

Ok(())
}

#[test]
fn test_insert_thousands_separator() {
assert_eq!(insert_thousands_separator("1234567.89"), "1,234,567.89");
assert_eq!(insert_thousands_separator("123.45"), "123.45");
assert_eq!(insert_thousands_separator("1234"), "1,234");
assert_eq!(insert_thousands_separator("12"), "12");
assert_eq!(insert_thousands_separator("0.5"), "0.5");
assert_eq!(
insert_thousands_separator("1234567890.1234"),
"1,234,567,890.1234"
);
assert_eq!(insert_thousands_separator("1000"), "1,000");
assert_eq!(insert_thousands_separator("100"), "100");
}

#[test]
fn test_grouping_separator_float() -> Result<()> {
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test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,.2f".to_string()))),
ColumnarValue::Scalar(ScalarValue::Float64(Some(1234567.89))),
],
Ok(Some("1,234,567.89")),
&str,
Utf8,
StringArray
);
Ok(())
}

#[test]
fn test_grouping_separator_decimal() -> Result<()> {
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,.2f".to_string()))),
ColumnarValue::Scalar(ScalarValue::Decimal128(Some(123456789), 10, 2)),
],
Ok(Some("1,234,567.89")),
&str,
Utf8,
StringArray
);
Ok(())
}

#[test]
fn test_grouping_separator_scientific_float() -> Result<()> {
// %,e — Java/Spark reject grouping separator with scientific notation
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,e".to_string()))),
ColumnarValue::Scalar(ScalarValue::Float64(Some(1234567.89))),
],
Err(DataFusionError::Execution(
"Grouping separator ',' flag is not compatible with scientific notation conversion 'e'".to_string(),
)),
&str,
Utf8,
StringArray
);
// %,E — uppercase scientific also rejected
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,E".to_string()))),
ColumnarValue::Scalar(ScalarValue::Float64(Some(1234567.89))),
],
Err(DataFusionError::Execution(
"Grouping separator ',' flag is not compatible with scientific notation conversion 'E'".to_string(),
)),
&str,
Utf8,
StringArray
);
// %,.0e — precision 0 scientific with grouping also rejected
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,.0e".to_string()))),
ColumnarValue::Scalar(ScalarValue::Float64(Some(1234567.89))),
],
Err(DataFusionError::Execution(
"Grouping separator ',' flag is not compatible with scientific notation conversion 'e'".to_string(),
)),
&str,
Utf8,
StringArray
);
Ok(())
}

#[test]
fn test_grouping_separator_compact_float() -> Result<()> {
// %,g with large number — triggers scientific, no commas
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,g".to_string()))),
ColumnarValue::Scalar(ScalarValue::Float64(Some(1234567.89))),
],
Ok(Some("1.23457e+06")),
&str,
Utf8,
StringArray
);
// %,g with small number — triggers fixed-point, commas in integer part
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,g".to_string()))),
ColumnarValue::Scalar(ScalarValue::Float64(Some(12345.6))),
],
Ok(Some("12,345.6")),
&str,
Utf8,
StringArray
);
// %,.0g — precision 0 compact with grouping (large number, scientific)
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,.0g".to_string()))),
ColumnarValue::Scalar(ScalarValue::Float64(Some(1234567.89))),
],
Ok(Some("1e+06")),
&str,
Utf8,
StringArray
);
// %,G — uppercase compact
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,G".to_string()))),
ColumnarValue::Scalar(ScalarValue::Float64(Some(1234567.89))),
],
Ok(Some("1.23457E+06")),
&str,
Utf8,
StringArray
);
Ok(())
}

#[test]
fn test_grouping_separator_scientific_decimal() -> Result<()> {
// %,e on decimal — Java/Spark reject grouping separator with scientific notation
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,e".to_string()))),
ColumnarValue::Scalar(ScalarValue::Decimal128(Some(123456789), 10, 2)),
],
Err(DataFusionError::Execution(
"Grouping separator ',' flag is not compatible with scientific notation conversion 'e'".to_string(),
)),
&str,
Utf8,
StringArray
);
// %,.0e on decimal — also rejected
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,.0e".to_string()))),
ColumnarValue::Scalar(ScalarValue::Decimal128(Some(123456789), 10, 2)),
],
Err(DataFusionError::Execution(
"Grouping separator ',' flag is not compatible with scientific notation conversion 'e'".to_string(),
)),
&str,
Utf8,
StringArray
);
Ok(())
}

#[test]
fn test_grouping_separator_compact_decimal() -> Result<()> {
// %,g on decimal — large number triggers scientific, no commas
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,g".to_string()))),
ColumnarValue::Scalar(ScalarValue::Decimal128(Some(123456789), 10, 2)),
],
Ok(Some("1.23457e+06")),
&str,
Utf8,
StringArray
);
// %,g on decimal — small number triggers fixed-point, commas expected
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,g".to_string()))),
ColumnarValue::Scalar(ScalarValue::Decimal128(Some(1234560), 10, 2)),
],
Ok(Some("12,345.6")),
&str,
Utf8,
StringArray
);
// %,.0g on decimal — precision 0 compact with grouping (scientific)
test_scalar_function!(
FormatStringFunc::new(),
vec![
ColumnarValue::Scalar(ScalarValue::Utf8(Some("%,.0g".to_string()))),
ColumnarValue::Scalar(ScalarValue::Decimal128(Some(123456789), 10, 2)),
],
Ok(Some("1e+06")),
&str,
Utf8,
StringArray
);
Ok(())
}
}
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