[I] Parsing zero scale numbers truncates instead of rounding the number [arrow-rs]

["gruuya (via GitHub)" <gi...@apache.org>]

gruuya opened a new issue, #9422:
URL: https://github.com/apache/arrow-rs/issues/9422

   **Describe the bug**
   The [fix](https://github.com/apache/arrow-rs/pull/8700) that corrected the decimal position spillover when parsing zero scale numbers also introduced another issue, which is that it always truncates the number instead of rounding it appropriately.
   
   **To Reproduce**
   The following test case fails, instead the parsed number is `1`.
   ```rust
   assert_eq!(
       parse_decimal::<Decimal128Type>("1.9", 3, 0).unwrap(),
       2,
   );
   ```
   
   **Expected behavior**
   It's not instantly clear that this should be the expected behavior in this case, but it would make sense to align with Postgres and DuckDB, where this is true
   ```sql
   postgres@localhost:postgres> select '1.1'::numeric(3, 0), '1.9'::numeric(3, 0);
   +---------+---------+
   | numeric | numeric |
   |---------+---------|
   | 1       | 2       |
   +---------+---------+
   ```
   
   ```sql
   D select '1.1'::numeric(3, 0), '1.9'::numeric(3, 0);
   ┌─────────────────────────────┬─────────────────────────────┐
   │ CAST('1.1' AS DECIMAL(3,0)) │ CAST('1.9' AS DECIMAL(3,0)) │
   │        decimal(3,0)         │        decimal(3,0)         │
   ├─────────────────────────────┼─────────────────────────────┤
   │                           1 │                           2 │
   └─────────────────────────────┴─────────────────────────────┘
   ```
   
   **Additional context**
   Note that the behavior is as expected when the decimal part is less than `.5`, since truncation and rounding then match up.


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Re: [I] Parsing zero scale numbers truncates instead of rounding the number [arrow-rs]

["Whatsonyourmind (via GitHub)" <gi...@apache.org>]

Whatsonyourmind commented on issue #9422:
URL: https://github.com/apache/arrow-rs/issues/9422#issuecomment-4182733753

   The fix needs to implement half-up rounding (banker's rounding or round-half-to-even would also be defensible, but Postgres and DuckDB both use half-up).
   
   The core logic: when parsing a decimal string with more fractional digits than the target scale allows, you need to look at the first truncated digit to decide whether to round up.
   
   ```rust
   // After parsing the significant digits and determining where truncation occurs:
   fn round_decimal_parse(digits: &[u8], truncation_point: usize, scale: i8) -> i128 {
       let mut result = parse_digits_up_to(digits, truncation_point);
       
       if truncation_point < digits.len() {
           let next_digit = digits[truncation_point] - b'0';
           if next_digit >= 5 {
               // Round away from zero (half-up for positive, half-down for negative)
               if result >= 0 {
                   result += 1;
               } else {
                   result -= 1;
               }
           }
       }
       result
   }
   ```
   
   For the specific case of scale=0, parsing "1.9" should give:
   1. Parse integer part: 1
   2. First truncated fractional digit: 9 >= 5, so round up
   3. Result: 2
   
   One edge case to watch: rounding can cause overflow. Parsing "99.5" as `numeric(2, 0)` should round to 100, which exceeds the precision. Postgres raises an error here. The implementation should check `result < 10^precision` after rounding and return an overflow error if violated.


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Re: [I] Parsing zero scale numbers truncates instead of rounding the number [arrow-rs]

["sdf-jkl (via GitHub)" <gi...@apache.org>]

sdf-jkl closed issue #9422: Parsing zero scale numbers truncates instead of rounding the number
URL: https://github.com/apache/arrow-rs/issues/9422


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Re: [I] Parsing zero scale numbers truncates instead of rounding the number [arrow-rs]

["neilconway (via GitHub)" <gi...@apache.org>]

neilconway commented on issue #9422:
URL: https://github.com/apache/arrow-rs/issues/9422#issuecomment-5375313142

   take


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Re: [I] Parsing zero scale numbers truncates instead of rounding the number [arrow-rs]

["sdf-jkl (via GitHub)" <gi...@apache.org>]

sdf-jkl commented on issue #9422:
URL: https://github.com/apache/arrow-rs/issues/9422#issuecomment-5916664180

   Closed via #10850.


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