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ARROW-10127: Update specification for Decimal to allow for 256-bits #8293

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ARROW-10127: Update specification for Decimal to allow for 256-bits #8293

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ARROW-10127: Update specification for Decimal to allow for 256-bits
emkornfield Sep 29, 2020
5cb139b
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emkornfield Sep 30, 2020
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32 changes: 17 additions & 15 deletions docs/source/format/CDataInterface.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -132,21 +132,23 @@ strings:
| ``g`` | float64 | |
+-----------------+--------------------------+------------+

+-----------------+---------------------------------------+------------+
| Format string | Arrow data type | Notes |
+=================+=======================================+============+
| ``z`` | binary | |
+-----------------+---------------------------------------+------------+
| ``Z`` | large binary | |
+-----------------+---------------------------------------+------------+
| ``u`` | utf-8 string | |
+-----------------+---------------------------------------+------------+
| ``U`` | large utf-8 string | |
+-----------------+---------------------------------------+------------+
| ``d:19,10`` | decimal128 [precision 19, scale 10] | |
+-----------------+---------------------------------------+------------+
| ``w:42`` | fixed-width binary [42 bytes] | |
+-----------------+---------------------------------------+------------+
+-----------------+---------------------------------------------------+------------+
| Format string | Arrow data type | Notes |
+=================+===================================================+============+
| ``z`` | binary | |
+-----------------+---------------------------------------------------+------------+
| ``Z`` | large binary | |
+-----------------+---------------------------------------------------+------------+
| ``u`` | utf-8 string | |
+-----------------+---------------------------------------------------+------------+
| ``U`` | large utf-8 string | |
+-----------------+---------------------------------------------------+------------+
| ``d:19,10`` | decimal128 [precision 19, scale 10] | |
+-----------------+---------------------------------------------------+------------+
| ``d:19,10,NNN`` | decimal bitwidth = NNN [precision 19, scale 10] | |
+-----------------+---------------------------------------------------+------------+
| ``w:42`` | fixed-width binary [42 bytes] | |
+-----------------+---------------------------------------------------+------------+

Temporal types have multi-character format strings starting with ``t``:

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10 changes: 4 additions & 6 deletions format/Schema.fbs
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Expand Up @@ -173,8 +173,8 @@ table Bool {
}

/// Exact decimal value represented as an integer value in two's
/// complement. Currently only 128-bit (16-byte) integers are used but this may
/// be expanded in the future. The representation uses the endianness indicated
/// 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
Expand All @@ -183,10 +183,8 @@ table Decimal {
/// Number of digits after the decimal point "."
scale: int;

/// Number of bits per value. The only accepted width right now is 128 but
/// this field exists for forward compatibility so that other bit widths may
/// be supported in future format versions. We use bitWidth for consistency
/// with Int::bitWidth.
/// Number of bits per value. The only accepted widths are 128 and 256.
/// We use bitWidth for consistency with Int::bitWidth.
bitWidth: int = 128;
}

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