Support multiple index statements in Field definitions

[Copilot]

This PR implements support for multiple index statements in Vespa Field definitions, addressing a limitation where fields could only have a single index configuration.

Problem

Previously, the schema template was hardcoded to render only one index statement per field:

index: {{ field.index }}

This prevented defining fields with complex index configurations like:

field predicate_field type predicate {
    indexing: attribute
    index {
        arity: 2
        lower-bound: 3
        upper-bound: 200
        dense-posting-list-threshold: 0.25
    }
}

Or fields requiring multiple index blocks with different configurations.

Solution

The implementation extends the Field class to accept multiple index configurations while maintaining full backward compatibility:

API Changes

The index parameter now accepts:

• String (existing): index="enable-bm25"
• Dict (existing): index={"arity": 2, "lower-bound": 3}
• List (new): index=["enable-bm25", {"arity": 2}, "another-setting"]

Examples

Single complex index block (from original issue):

Field(
    name="predicate_field",
    type="predicate", 
    indexing=["attribute"],
    index={
        "arity": 2,
        "lower-bound": 3,
        "upper-bound": 200,
        "dense-posting-list-threshold": 0.25
    }
)

Multiple index configurations:

Field(
    name="complex_field",
    type="string",
    index=[
        "enable-bm25",              # Simple setting
        {"arity": 2, "lower-bound": 3},  # Complex block
        "another-setting"           # Another simple setting
    ]
)

Parameterless index settings:

Field(
    name="taxonomy", 
    type="array<string>",
    index={"enable-bm25": None}  # Renders as just "enable-bm25"
)

Template Updates

Updated Jinja2 templates to iterate over multiple index configurations and handle parameterless settings (None values render without : None).

Backward Compatibility

All existing code continues to work unchanged. The new functionality is additive and doesn't break any existing Field definitions.

Testing

Added comprehensive test suite with 11 test cases covering:

• Backward compatibility verification
• Multiple index configuration scenarios
• Mixed string/dict configurations
• ANN field integration
• Edge case handling

Fixes #983.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.

[thomasht86]

Good bot! 🤖

[thomasht86]

@kkraune need another approval since I collaborated with Copilot on this one. 🙏

[kkraune]

nice work by the robot!