RFC 019: Window functions

[dannymeijer]

Use this issue to track InQL RFC 019, which already exists at docs/rfcs/019_window_functions.md.

Area

• Specification (RFCs)
• Package & tests
• Documentation

Summary

RFC 019 defines InQL window functions and window specifications: partitioning, ordering, frames, ranking functions, offset functions, and value functions.

Motivation

Window functions are not ordinary aggregates. They produce one value per input row while seeing a related set of rows, so InQL needs a distinct placement, typing, and lowering contract.

Proposal sketch

The RFC defines window function classes and window specification rules for query-block and dataframe-style authoring. It separates analytic windows from aggregate semantics and from future streaming event-time windows.

Open design questions to resolve before Planned:

• What default frame should InQL use for ordered window functions?
• Should window functions be allowed in WHERE or only in projection/order positions?
• Should null treatment use explicit IGNORE NULLS / RESPECT NULLS style modifiers?

Alternatives considered

The RFC rejects treating windows as aggregates, delaying window functions until streaming windows exist, and exposing only backend SQL strings for windows.

Impact / compatibility

This RFC affects window helper APIs, query syntax checking, placement diagnostics, Prism/Substrait lowering for partition/order/frame semantics, and documentation that separates aggregate and window functions.

Implementation notes (optional)

Handle after the registry and relevant aggregate/scalar foundations are settled so window function entries share the same function identity and metadata model.

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).