[DISCUSSION] How to represents decimal type?

[mapleFU]

For decimal type, iceberg-rust[1] uses rust_decimal[2] crate, and iceberg-java uses Java's BigDecimal[3]. iceberg-go uses a hand-written decimal [4]

We can uses a hand-written decimal in Literal. Or uses arrow's implemetation?

[1] https://github.com/apache/iceberg-rust/commits/e5d3aabf1a15892e89eca7850e32720d89e65e7c/crates/iceberg/src/spec/values.rs
[2] https://docs.rs/rust_decimal/latest/rust_decimal/
[3] https://github.com/apache/iceberg/blob/12605226736312970d787e704d4b7352a2bc2c1c/api/src/main/java/org/apache/iceberg/expressions/Literal.java#L22
[4] https://github.com/apache/iceberg-go/blob/ff2f3de32af5291ef4a35ce79978bb8000a9a5ff/types.go#L486

[wgtmac]

We cannot use Arrow's decimal impl because libiceberg does not depend on Arrow but only nanoarrow.

[zhjwpku]

After experimenting on Godbolt, I found that using the following declaration works well to represent int128_t. However, when trying to build Decimal128 with it, I realized that adapting certain decimal functions becomes difficult and the implementation is prone to bugs. Therefore, I decide to follow Arrow's approach and extract the Decimal128 implementation from Arrow into iceberg-cpp. Adoption of int128_t can come later if we found it necessary.

#if defined(_MSC_VER)
#  include <__msvc_int128.hpp>
using int128_t = ::std::_Signed128;
using uint128_t = ::std::_Unsigned128;
#elif defined(__GNUC__) || defined(__clang__)
using int128_t = __int128;
using uint128_t = unsigned __int128;
#else
#  error "128-bit integer type is not supported on this platform"
#endif

[wgtmac]

Agreed. We can use the battle-tested implementation from Arrow for now and switch to other alternatives if needed.

[zhjwpku]

#182 has been merged into main, so this issue can now be closed. I'll be working on adding Literal support for Decimal in the coming days, thanks.