perf: store small decimal values as int

[wangrunji0408]

RisingLight now uses rust_decimal to represent decimal values. However, computations on this type are relatively slow due to its large precision (96 bits) and variable scale. For small decimals, e.g. DECIMAL(15,2) in TPC-H tables, we could store and compute them as i64. This would bring a huge performance boost (considering SIMD). A potential challenge of this change is that we can no longer tell the exact value from array without its datatype. 🤔

Reference: https://duckdb.org/docs/sql/data_types/numeric#fixed-point-decimals

[unconsolable]

IMHO another challenge is converting arrays from different WIDTH/SCALE to perform arithmetic, comparison, etc... Seems that DuckDB puts some efforts on it, while arrow-rs don't consider it.

ref. apache/arrow-rs#2637, duckdb/duckdb#858