Case-insensitive field matching in struct casting

[kosiew]

Problem

This is a design discussion issue regarding whether DataFusion should adopt case-insensitive field matching when casting between structs.

Current Behavior

DataFusion uses case-sensitive field name matching. For example:

• Field x and field X are treated as different fields
• A cast from struct<x int, y int> to struct<X int, Y int> would fail (no name overlap)

DuckDB Behavior

DuckDB uses case-insensitive field name matching:

• Field x and field X are treated as the same field
• The same cast would succeed, with fields matched case-insensitively

Motivation for Case-Insensitive Matching

Pros:

• ✅ Aligns with DuckDB — improves compatibility with a major SQL database
• ✅ More forgiving — handles common casing variations (e.g., JSON sources with inconsistent field names)
• ✅ Follows SQL conventions — SQL generally treats identifiers as case-insensitive
• ✅ User-friendly — reduces friction when working with data from different sources

Arguments for Keeping Case-Sensitive Matching

Pros:

• ✅ Arrow foundation — DataFusion is built on Apache Arrow, which is case-sensitive:
• ✅ Language consistency — matches Rust and JSON conventions (case-sensitive)
• ✅ Prevents ambiguity — avoids edge cases where source has both x and X (rare but possible)
• ✅ Predictable behavior — case-sensitive matching is more explicit and easier to reason about in programmatic contexts

Question

Should DataFusion follow SQL's case-insensitivity or remain aligned with Arrow's case-sensitive semantics?

Next Steps

This issue is intended to surface the design question and gather community feedback.