RFC 021: Generator and table-valued functions

[dannymeijer]

Use this issue to track InQL RFC 021, which already exists at docs/rfcs/021_generator_table_functions.md.

Area

• Specification (RFCs)
• Package & tests
• Documentation

Summary

RFC 021 defines generator and table-valued functions for InQL, including explode, explode_outer, posexplode, posexplode_outer, inline, inline_outer, flatten, stack, and selected tuple-producing extraction helpers.

Motivation

These functions change relation shape or cardinality, so they must be modeled as relation operations rather than scalar expressions.

Proposal sketch

The RFC defines generator and table-valued function boundaries, placement rules, output aliasing, schema effects, and compatibility limits. It keeps relation-shaping functions distinct from nested scalar functions in RFC 020.

Open design questions to resolve before Planned:

• Should positional generators use zero-based or one-based positions?
• Should .explode(...) preserve all input columns by default?
• What aliasing syntax should be required for generated output columns?
• What subset of Snowflake-style flatten behavior belongs in portable InQL versus a warehouse compatibility extension?

Alternatives considered

The RFC rejects modeling generators as scalar functions returning arrays, allowing generators anywhere a scalar expression is allowed, and supporting only explode without settling the broader class boundary.

Impact / compatibility

This RFC affects relation-shaping APIs, query syntax placement checks, schema updates after generator use, Prism/Substrait lowering for cardinality changes, and docs explaining generator cardinality/schema effects.

Implementation notes (optional)

Coordinate with RFC 020 so nested scalar operations and generator/table-valued operations remain separate and diagnostically clear.

Checklist

• I checked for an existing RFC or issue covering this.
• I can describe how this impacts existing code and how to migrate (if needed).