[jira] [Assigned] (ARROW-15159) [C++] Cast from integer to decimal to check the required precision

["Quang Hoang (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-15159?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Quang Hoang reassigned ARROW-15159:
-----------------------------------

    Assignee: Quang Hoang

> [C++] Cast from integer to decimal to check the required precision
> ------------------------------------------------------------------
>
>                 Key: ARROW-15159
>                 URL: https://issues.apache.org/jira/browse/ARROW-15159
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>            Reporter: Dragoș Moldovan-Grünfeld
>            Assignee: Quang Hoang
>            Priority: Major
>              Labels: good-first-issue, kernel
>
> Casting from integer to decimal currently requires a precision of at least 12 (I think that might be the maximum precision required to represent any {{{}int32{}}}), irrespective of the actual values requiring casting.
> An example of how this surfaces in R:
> {code:r}
> library(arrow)
> a <- as.double(1:10)
> b <- 1:10
> typeof(a)
> #> [1] "double"
> typeof(b)
> #> [1] "integer"
> Array$create(a)$cast(decimal(4, 2))
> #> Array
> #> <decimal128(4, 2)>
> #> [
> #>   1.00,
> #>   2.00,
> #>   3.00,
> #>   4.00,
> #>   5.00,
> #>   6.00,
> #>   7.00,
> #>   8.00,
> #>   9.00,
> #>   10.00
> #> ]
> # a precision of 4 should be more than enough for this example
> Array$create(b)$cast(decimal(4, 2))
> #> Error: Invalid: Precision is not great enough for the result. It should be at least 12
> {code}
> Float to decimal casting checks each value instead of upfront. [See here|https://github.com/apache/arrow/blob/f473cb908f4b25e3d5e4343af5c4b567751c0995/cpp/src/arrow/compute/kernels/scalar_cast_numeric.cc#L546-L578].



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