[spec/drivers] AggregationNode.distinct is honoured by the in-memory fallback and ignored by every SQL face — one query, two numbers (ADR-0049) #6815

Filed unassigned from the #6409 lane. Found while lowering count_distinct; adjacent to it, not created by it.

The finding

AggregationNodeSchema (packages/spec/src/data/query.zod.ts) declares a per-aggregation flag:

distinct: z.boolean().optional().describe('Apply DISTINCT before aggregation'),

Exactly one consumer reads it. packages/objectql/src/in-memory-aggregation.ts:

const values = collectValues(rows, field, !!agg.distinct);

Nothing else in the repo does — measured by grepping every consumer of aggregations[]:

So { function: 'sum', field: 'amount', distinct: true } returns a deduplicated sum when the engine falls back in memory and an ordinary sum on every SQL datasource. Same query, two numbers, chosen by which backend answered — the divergence class #6203 and #5907 each closed on the aggregate axis, still open on this key. And unlike those, the wrong answer here is a plausible number rather than a refusal, so nothing surfaces it.

The key was NOT covered by #4286, which swept the request surface: that issue dispositioned QueryAST.distinct (the query-level one, since retired) and AggregationNode.filter (marked [EXPERIMENTAL — not enforced]). AggregationNode.distinct is neither — it is declared plainly, has a real consumer, and reads as supported.

Why it is worth a decision now rather than later

#6409 lowered count_distinct on the SQL family, which changes what this key looks like to an author. count_distinct is now a working, portable, deduplicated count; distinct: true beside it is the affordance that reads as "…and the same for sum/avg". One of the two now works everywhere and the other works in one place, which is a sharper false affordance than it was yesterday.

Two legs, per ADR-0049

ENFORCE. SELECT SUM(DISTINCT col) is standard SQL and supported by SQLite, PostgreSQL and MySQL, so the lowering exists and is portable — the same argument that kept count_distinct declared at #6188. driver-sql's and driver-turso's lowering tables already carry a distinct flag as of #6409, so the emitters have the shape; what they lack is reading it off the NODE as well as off the function. driver-memory/driver-mongodb are #5499-frozen, which is the usual constraint on this leg.

REMOVE. count_distinct covers the only spelling anyone has measured demand for, SUM(DISTINCT …) / AVG(DISTINCT …) are near-universally a modelling mistake, and the key has no stored-metadata authoring surface — QueryAST is the client SDK builder's output and the POST /data/:object/query body, so there is no conversion to write (the #4286 note on the request surface applies verbatim). Removal costs the in-memory fallback its collectValues flag and nothing else.

No recommendation offered — this is the maintainer's call, in the same shape as #6188's.

What is NOT in scope of this issue

count(distinct *) and friends: count_distinct with no field is refused with INVALID_QUERY / 400 as of #6409, on both SQL faces. Separately, sum/avg/min/max written with no field still emit sum(*) and die as a dialect syntax error with no ADR-0112 envelope — a real but distinct gap, and one #6409 deliberately did not widen its refusal surface to cover.

Refs: #6409, #6188, #4286, ADR-0049, #5499.