fix(udf): fix decimal values

[wangrunji0408]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

This PR fixes #11823.

The problem is that Arrow has no data type equivalent to the DECIMAL (unconstrained numeric) in RisingWave/PostgreSQL. The decimal types in Arrow are fixed-point, while our decimal type is floating-point. Previously we used decimal(38,0) in arrow to represent decimal values. This is clearly wrong because it only accepts integer numbers.

This PR switches the arrow type for decimal to "large-binary". Values are stored as their string representations in the array, like [b"1.23", b"0", b"-inf"]. This may not be efficient in encoding, but is least error prone. And theoretically, this design can accept arbitrary-precision decimal values.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

Documentation

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Release note

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[liurenjie1024]

This solution is too hacky to me. Converting to arrow not only used in udf, but also used in inserting to iceberg/parquet.

[wangrunji0408]

This solution is too hacky to me. Converting to arrow not only used in udf, but also used in inserting to iceberg/parquet.

But arrow does not have a type for floating-point decimal. It's impossible to use a fixed type decimal array to store them.

[liurenjie1024]

This solution is too hacky to me. Converting to arrow not only used in udf, but also used in inserting to iceberg/parquet.

But arrow does not have a type for floating-point decimal. It's impossible to use a fixed type decimal array to store them.

Yeah, that's a problem. I think we need a discussion before deciding a solution.

[github-actions]

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[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (da79ff5) 68.15% compared to head (9964686) 68.13%.

Files	Patch %	Lines
src/common/src/array/arrow.rs	69.23%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11839      +/-   ##
==========================================
- Coverage   68.15%   68.13%   -0.03%     
==========================================
  Files        1524     1524              
  Lines      262331   262309      -22     
==========================================
- Hits       178793   178721      -72     
- Misses      83538    83588      +50     

Flag	Coverage Δ
rust	68.13% <69.23%> (-0.03%)	⬇️

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[liurenjie1024]

LGTM, it's better to add some comments to explain it.

[wangrunji0408]

This change breaks decimal handling in iceberg. Should be fixed in the future.