feat: delimiter for agg_concat

[aaron-ang]

Changes Made

Related Issues

Closes #2959.

[greptile-apps]

Greptile Overview

Greptile Summary

This PR adds an optional delimiter parameter to the agg_concat/string_agg aggregation function, allowing users to specify a custom separator when concatenating string values. The feature addresses issue #2959.

Key Changes:

• Added optional delimiter: str | None parameter to Python API (string_agg, agg_concat methods)
• Updated Rust implementation to handle delimiter in AggExpr::Concat variant
• Implemented custom string joining logic using join_with_delimiter helper function
• Delimiter only supported for Utf8 columns; returns error if used with List columns
• Empty delimiter strings are filtered out (treated as None)
• Multi-stage aggregation properly propagates delimiter through all stages

Implementation Details:

• The delimiter is filtered at the Rust DSL level to treat empty strings as None
• Type checking ensures delimiter is only used with Utf8 columns
• Null values are properly filtered out during concatenation
• Both global and grouped aggregations support the delimiter parameter

Test Coverage:
Added tests for delimiter functionality in both global and grouped aggregation scenarios.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The implementation is well-structured with proper type checking, error handling for invalid delimiter usage, consistent propagation through all layers (Python, Rust DSL, execution), and includes test coverage for the new functionality
• No files require special attention

Important Files Changed

Filename	Overview
daft/functions/agg.py	Updated string_agg function to accept and pass through delimiter parameter
src/daft-core/src/series/ops/agg.rs	Implemented custom string concatenation with delimiter support, includes proper null handling
src/daft-dsl/src/expr/mod.rs	Added delimiter field to AggExpr::Concat, updated all related methods and type inference
tests/recordbatch/test_table_aggs.py	Added tests for delimiter parameter in both global and grouped aggregations

Sequence Diagram

sequenceDiagram
    participant User
    participant Python API
    participant PyExpr
    participant Rust DSL
    participant RecordBatch
    participant Series
    
    User->>Python API: df.agg(col("a").string_agg(delimiter=" "))
    Python API->>PyExpr: agg_concat(delimiter=" ")
    PyExpr->>Rust DSL: Expr::agg_concat(delimiter=Some(" "))
    Note over Rust DSL: Filter empty delimiter strings<br/>Store as AggExpr::Concat(expr, delimiter)
    Rust DSL->>RecordBatch: eval_agg_expr(AggExpr::Concat)
    RecordBatch->>Series: agg_concat(groups, delimiter)
    
    alt Delimiter provided for Utf8
        Series->>Series: join_with_delimiter(iter, delimiter)
        Note over Series: Custom implementation:<br/>- Filter nulls<br/>- Join with delimiter
    else No delimiter or List type
        Series->>Series: DaftConcatAggable::concat()
        Note over Series: Use existing trait implementation
    end
    
    Series-->>RecordBatch: Concatenated result
    RecordBatch-->>Rust DSL: Series
    Rust DSL-->>Python API: PyExpr
    Python API-->>User: DataFrame with aggregated column

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[codecov]

Codecov Report

❌ Patch coverage is 7.21649% with 90 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.39%. Comparing base (aa8add2) to head (61e333a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/daft-core/src/series/ops/agg.rs	0.00%	56 Missing ⚠️
src/daft-dsl/src/expr/mod.rs	0.00%	17 Missing ⚠️
src/daft-local-plan/src/agg.rs	0.00%	7 Missing ⚠️
src/daft-logical-plan/src/ops/project.rs	0.00%	4 Missing ⚠️
src/daft-dsl/src/expr/agg.rs	0.00%	2 Missing ⚠️
src/daft-recordbatch/src/lib.rs	0.00%	2 Missing ⚠️
daft/dataframe/dataframe.py	75.00%	1 Missing ⚠️
src/daft-sql/src/modules/aggs.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6099       +/-   ##
===========================================
- Coverage   72.91%   43.39%   -29.52%     
===========================================
  Files         973      909       -64     
  Lines      126196   112787    -13409     
===========================================
- Hits        92016    48947    -43069     
- Misses      34180    63840    +29660     

Files with missing lines	Coverage Δ
daft/expressions/expressions.py	95.12% <100.00%> (ø)
daft/functions/agg.py	97.61% <100.00%> (ø)
daft/dataframe/dataframe.py	77.38% <75.00%> (ø)
src/daft-sql/src/modules/aggs.rs	37.28% <0.00%> (-33.06%)	⬇️
src/daft-dsl/src/expr/agg.rs	0.00% <0.00%> (-77.56%)	⬇️
src/daft-recordbatch/src/lib.rs	21.62% <0.00%> (-53.06%)	⬇️
src/daft-logical-plan/src/ops/project.rs	45.97% <0.00%> (-10.64%)	⬇️
src/daft-local-plan/src/agg.rs	0.00% <0.00%> (-93.50%)	⬇️
src/daft-dsl/src/expr/mod.rs	39.29% <0.00%> (-35.62%)	⬇️
src/daft-core/src/series/ops/agg.rs	2.64% <0.00%> (-66.89%)	⬇️

... and 646 files with indirect coverage changes

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