Add tickets and ideas

2026-09-07 12:16:36 +01:00
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+# 098 — No histogram_quantile()-style percentile query
+
+## Summary
+
+The PromQL layer's `histogram()` op buckets raw sample values by a
+fixed width and returns per-bucket counts — it's unrelated to
+Prometheus's classic histogram bucket/quantile model, and there's no
+way to ask for p50/p95/p99 out of a metric today.
+
+## Where
+
+- `src/engine/promql.rs:9-16` — `AggOp::Histogram` is one of the six
+  supported ops.
+- `src/engine/snapshot/mod.rs:1002` onward — `Histogram` evaluation:
+  buckets values by `bucket_width` into counts, grouped by
+  `by_labels`. This answers "how many samples fell in each bucket",
+  not "what value is the Nth percentile".
+- No `histogram_quantile()` function or equivalent exists anywhere in
+  `promql.rs`'s parser (`parse_query`, `:261-324` recognizes only
+  `sum`, `avg`, `count`, `rate`, `histogram`, `gauge`).
+- Depends on ticket 097 (gauge/counter semantics) for the broader
+  question of whether a metric needs a declared "this holds raw
+  observations for quantile purposes" flag, but quantile computation
+  itself doesn't strictly require that — it can operate on whatever
+  raw values are already stored in a metric's buckets.
+
+## Scope
+
+- Decide the quantile computation model: interpolate over the raw
+  stored sample values in a bucket/window (simplest, exact for the
+  data actually retained) vs. building a genuine bucketed-histogram
+  representation with configurable bucket boundaries (closer to
+  Prometheus's classic histograms, more work, needed only if retained
+  raw values are later dropped in favor of pre-aggregated buckets).
+- Add a `histogram_quantile(p, metric[range])`-shaped PromQL construct
+  (or extend `histogram()` with a quantile argument) to
+  `promql.rs`'s parser and `AggOp`, plus an evaluator in
+  `snapshot/mod.rs` alongside the existing `Histogram` handling.
+- Add dashboard/panel UI support for selecting a quantile once the
+  query-language side exists (`webapp/src/components/Dashboards.tsx`
+  or wherever panel query construction lives).