isthmus: support struct field access from RexLambdaRef

[vinaykpud]

Problem

Isthmus cannot convert struct-field access rooted at a Calcite RexLambdaRef.

For an array of structs:

transform(events, event -> event.name)

the canonical Calcite lambda has one parameter representing the array element:

lambda(event: ROW<name, attributes>) -> event.name

However, converting RexFieldAccess(RexLambdaRef(event), name) fails with:

UnsupportedOperationException:
RexFieldAccess for SqlKind LAMBDA_REF not supported

Observed with 0.89.1 and current main at ade72bc2.

Cause

RexExpressionConverter.visitLambda() and visitLambdaRef() support lambda expressions and references, but visitFieldAccess() only accepts:

• ITEM
• INPUT_REF
• FIELD_ACCESS
• CORREL_VARIABLE

It does not accept LAMBDA_REF.

This prevents canonical list-function lambdas over struct elements. Consumers must instead model every struct child as a separate lambda parameter and repair the generated Substrait protobuf afterward.

Expected behavior

Isthmus should convert:

RexFieldAccess(RexLambdaRef(event), name)

into a lambda parameter field reference equivalent to:

lambda_parameter[0].field[0]

Multi-level access should also work:

employee.profile.address.city

Suggested changes

1. Handle SqlKind.LAMBDA_REF in RexExpressionConverter.visitFieldAccess().
2. Preserve lambda-reference scope when calling FieldReference.dereferenceStruct().
3. Support chained field segments for LAMBDA_PARAMETER_REFERENCE in ProtoExpressionConverter.
4. Add forward and round-trip tests for:
   • employee.name
   • employee.profile.address.city

Related work

• isthmus: lambda support #689 — Isthmus lambda-support umbrella
• isthmus: Lambda support limitation #711 — nested lambda scopes; this issue concerns steps_out = 0
• Support SQL to Substrait conversion for List functions in Isthmus #757 / fix(isthmus): support SQL list function lambdas #1305 — SQL list-function lambda inference
• fix(core): retain reference scope during dereference #1268 — preserves scoped-reference metadata during dereference