SQL: ClickHouse's argMax has no AggIntent representation #232

Summary

The bgp_jan2024_workload 200-query corpus (#219) has 3 occurrences of ClickHouse's argMax(arg, val) — "return arg's value from the row where val is maximal" — which currently fails to lower. Unlike #230's scalar-function gaps, this isn't a planner-registration problem to route around: there's no existing AggIntent variant (crates/types/src/pre_asap/agg_intent.rs) that can represent it at all.

Why this doesn't fit the existing shape

AggIntent::Max { col } reduces one column to its own extreme value. argMax(arg, val) is different in kind: it returns a different column's value, selected by which row maximizes a second column — a two-column, row-selecting aggregate, not a single-column reduction. None of the existing variants (Min/Max/Sum/Avg/Quantile/TopK/Cardinality/...) have this shape.

Two ways to represent it

1. A proper new core AggIntent::ArgMax { arg_col, val_col } variant (and presumably ArgMin alongside it, and their by-group/schema-derivation logic in aggregate_output_schema) — if this is common enough across dialects/deployment models to earn a first-class spot.
2. AggIntent::Extension — the crate's own deliberate escape hatch for exactly this situation ("a deployment model that needs something core doesn't have carries it here, tagged by its own kind string... core only grows for intents ≥2 deployment models actually use," per its doc comment). Core would treat it opaquely (no schema derivation, no per-series/data-model logic) until/unless a second use case justifies promoting it to (1).

Scope of this issue

Settle which of the two directions to take (or gather more evidence — e.g. does any PromQL-side or another SQL dialect's workload also want arg-max/arg-min, which would tip the scale toward a first-class variant per the crate's own "≥2 deployment models" bar?). Not asking for an implementation yet.

Related

• feat: add bgp_jan2024_workload SQL corpus test (200 queries, ASAPQuery PR #561) #219 — bgp_jan2024_workload corpus test, where this gap was found
• SQL: generalize the ClickHouse-builtin stub-registration mechanism from AggregateUDF to ScalarUDF #230 — the sibling scalar-function catalog issue this is not the same shape as
• crates/types/src/pre_asap/agg_intent.rs — AggIntent, AggIntent::Extension