Grouping: census of shapes still served by the legacy ladder

Status: recorded 2026-09-19 after the core-scaling follow-up. The parallel grouping engine ("v2") now owns every top-N emit filter, unordered bounded emit, multi-key arithmetic over aggregates, and composite symbol keys. This census lists what still reaches the legacy grouping ladder in exec_group_run, as the factual starting point for retiring it.

How it was measured

The test driver records, for every evaluated .rfl line, whether the grouping executor took the legacy route and the admission reason it was given:

make test                     # builds ./rayforce.test
RAYFORCE_CORES=2 ./rayforce.test --census census.tsv -f rfl/
cut -f1 census.tsv | sort | uniq -c

Each census line is reason<TAB>file:line<TAB>source. Run it with the suite's worker count: one admission reason (parallel_wide) depends on the pool size, and several ordering assertions in the corpus assume two workers.

Results (514 .rfl files, 340 legacy hits)

Reason	Lines	With by:	What it means	What v2 needs
shape	200	5	195 are scalar aggregations with no keys (select {s: (sum v) from: T}), mostly over partitioned stores; the 5 grouped ones are nested selects whose outer scalar select carries the reason, plus one by: on a missing column	A keyless (single-group) path in v2, or routing scalar aggregation to the vector aggregators; the grouped 5 are already served
agg_expression	63	63	Aggregate over an expression: (sum (strlen s)) ×35, (sum (at ...)) ×15, (first (at ...)) ×14, (avg (strlen s)) ×11, (count (select ...)) ×7	The expression-input path (exec_group_v2_exprs) materializes inputs it can evaluate as a full column; string-length and indexed/nested inputs are declined today
key_expression	31	27	Computed keys: (+ k 1) ×10, (xbar ts N) ×5, (substr ref 0 3) ×2, (minute EventTime), dotted temporal accessors ts.date	Materialize computed keys as scan columns before admission, the way expression aggregate inputs are; temporal accessors need the same
parallel_wide	30	30	Float keys with 2 to 8 workers and no indexed aggregate (by: {f: f} on F64) — a measured choice to prefer the ladder's directory over v2's replicated float hash	Make the v2 float route competitive at small pools (or drop the pool-size gate once measured)
admitted	16	15	Admitted by v2 but executed on the ladder: partitioned-store sources (from: Pmc by: date) where the per-partition executor merges partial groups on the legacy path, and one derived-key symbol-domain case	A partition-aware merge in v2 (streaming states merge; buffered ones need the ladder's row slices)

No line reached the ladder because of an emit filter, a bounded emit, or a multi-key compound expression; those classes are closed.

Reading the table

• Two classes are not grouping at all: keyless scalar aggregation (195 lines) and the outer scalar select of a nested query. They keep the ladder alive for reasons unrelated to group-by scaling and can be moved independently.
• The grouped remainder is 140 lines in four classes. Three of them (agg_expression, key_expression, admitted on partitioned sources) are admission gaps that a materialize-then-scan step closes without a new strategy; parallel_wide is a performance gate that needs a measurement, not a feature.
• The benchmark query files add two shapes the corpus lacks: a computed key through several nested string functions, and a where: filter on the grouped result ((> c 100000)), which the emit filter already expresses.

Next step

Retiring the ladder is its own plan: close the four grouped classes above, route keyless aggregation to the vector aggregators, then delete exec_group_run and the consumers it alone uses.