[GitHub] [arrow] ovr commented on pull request #9232: ARROW-10818: [Rust] Implement DecimalType

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

ovr commented on pull request #9232:
URL: https://github.com/apache/arrow/pull/9232#issuecomment-766338705


   > Looking at the spec, a decimal type only supports 128 and 256 bits. So, I am not understanding why we are trying to add support for Int32,Int64,Int128,LargeDecimal here.
   
   Arrow is used inside DF, which is used to build databases on to of it. If the user defines `DECIMAL(2,2)` it's a bit overhead to use i128 as representation for it.
   
   1. Memory usage, `DecimalArray<Decimal128>` will use 16 bytes for each element.
   2. Performance, Rust is using LLVM, which allows integers of any bit width from 1 to 2^23. Inside LLVM backend for x86-64, It will use rax, rcx to store it.
   
   It's ok to store DECIMAL(2,2) as Int64 inside parquet (but anyway overhead), but it's not great to use it as memory type.


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