[GitHub] [arrow-rs] jorgecarleitao commented on issue #1779: DataType::Decimal Non-Compliant

[GitBox <gi...@apache.org>]

jorgecarleitao commented on issue #1779:
URL: https://github.com/apache/arrow-rs/issues/1779#issuecomment-1146037480

   Sorry, I am probably misreading this - I am not following what is the non-compliance.
   
   The Python documentation exemplifies:
   
   > As an example, decimal128(7, 3) can exactly represent the numbers 1234.567 and -1234.567 (encoded internally as the 128-bit integers 1234567 and -1234567, respectively)
   
   (so, 7 digits, 3 of them are decimal)
   
   we offer the example
   
   > For example the number 123.45 has precision 5 and scale 2.
   
   The spec is https://github.com/apache/arrow/blob/master/format/Schema.fbs#L185 (copied for readability below)
   
   ```
   /// Exact decimal value represented as an integer value in two's
   /// complement. Currently only 128-bit (16-byte) and 256-bit (32-byte) integers
   /// are used. The representation uses the endianness indicated
   /// in the Schema.
   table Decimal {
     /// Total number of decimal digits
     precision: int;
   
     /// Number of digits after the decimal point "."
     scale: int;
   
     /// Number of bits per value. The only accepted widths are 128 and 256.
     /// We use bitWidth for consistency with Int::bitWidth.
     bitWidth: int = 128;
   }
   ```
   
   Aren't they all aligned?
   
   > In particular with the current rust definition it is unclear how to represent numbers with more than 38 digits, either because of leading or trailing 0s.
   
   The Python docs continue
   
   > [Decimal128Type](https://arrow.apache.org/docs/cpp/api/datatype.html?highlight=decimal#classarrow_1_1_decimal128_type) has a maximum precision of 38 significant digits
   
   which also seems aligned?


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