feat(sketchdb): surface accuracy envelope on PromQL response (§6.4, A+C) - #56
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Addresses TODO.md blocker #2 subitem #3. Queries answered by the sketch DB now carry a theoretical accuracy bound back to the client as two fields on the Prometheus HTTP response: * **A — structured `accuracy` (top-level)**: ```json "accuracy": { "epsilon": 0.008125, "delta": 0.0, "kind": "relative_cardinality", "per_segment": [...] // populated on schema-timeline crossings } ``` * **C — human-readable `infos` (Prometheus 3.0 / Grafana 11+)**: ```json "infos": ["accuracy: ε=0.008125, δ=0, kind=relative_cardinality"] ``` Both are standard Prometheus-tolerant extensions — unknown top-level fields are ignored by the upstream client/Grafana 10. A regression test confirms a stripped-down "standard Prometheus" decoder round-trips through our extended response. ## Wire shape `warnings` stays reserved for partial-result / fallback advisories (PR #49's schema-timeline dispatcher still uses it); accuracy gets its own dedicated field so the semantics don't mix. When a query crosses a schema-timeline boundary, `per_segment` lists each segment's `(agg_id, range_ms, profile)`. The top-level `profile` is the max-ε, max-δ envelope across segments — a conservative upper bound. ## Changes * `stores/sketch_db/accuracy.rs`: * `AccuracyEnvelope { profile, per_segment }` + builders (`single`, `from_segments`) * `PerSegmentAccuracy { agg_id, range_ms, profile }` * `AccuracyProfile::summary()` / `AccuracyEnvelope::summary()` emit the `infos` one-liner. * `engines/query_result.rs`: `InstantVector` / `RangeVector` carry `accuracy: Option<AccuracyEnvelope>`; new `QueryResult::{accuracy(), with_accuracy()}`. * `drivers/query/adapters/prometheus_http.rs`: `PrometheusResponse::{infos, accuracy}` fields + `with_accuracy()` builder. `format_success_response` / `format_range_success_response` thread `result.accuracy()` onto the response. * `engines/simple_engine.rs`: * `SimpleEngine::accuracy_envelope_for(agg_id)` — single- aggregation helper. * `execute_context` attaches single-agg accuracy. * Timeline dispatch builds per-segment accuracy list and attaches the multi-segment envelope. ## Tests 777 → 784 (+5 green): * `prometheus_response_carries_accuracy_top_level_and_infos_mirror` * `prometheus_response_without_accuracy_skips_both_fields` * `prometheus_response_per_segment_contains_all_segments_with_worst_case_top` * `accuracy_coexists_with_warnings_without_interference` * `promql_standard_client_can_decode_response_ignoring_extensions` clippy + fmt clean.
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Addresses TODO.md blocker #2 sub-item #3. Queries answered by the sketch DB now carry a theoretical accuracy bound back to the client as two fields on the Prometheus HTTP response:
A — structured `accuracy` (top-level)
```json
"accuracy": {
"epsilon": 0.008125,
"delta": 0.0,
"kind": "relative_cardinality",
"per_segment": [ /* populated on schema-timeline crossings */ ]
}
```
C — human-readable `infos` (Prometheus 3.0 / Grafana 11+)
```json
"infos": ["accuracy: ε=0.008125, δ=0, kind=relative_cardinality"]
```
Compatibility
warnings#49); accuracy doesn't hijack it.Cross-boundary semantics
When a query crosses a schema-timeline reconfigure, `per_segment` lists each piece's `(agg_id, range_ms, profile)`; the top-level envelope is (max ε, max δ) — a conservative upper bound across segments.
Tests
777 → 784 (+5), clippy + fmt clean.
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