258 add capability based matching to asap query engine instead of matching incoming queries against pre configured queries in inference config

.docs/CAPABILITY_MATCHING_DESIGN.md

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+# Capability-Based Aggregation Matching — Design Decision Record
+
+## Problem Statement
+
+Currntly, when query engine gets a query, it does an exact string match of that query against the inference config to find what aggregation ID to use. This results in 2 problems:
+1. It also requires the inference config to exactly specify each and every query that can be supported.
+2. This does NOT handle ad-hoc queries for which we may still have a sketch that is computed.
+
+For instance, even when the same sketch can answer both `quantile(0.5, metric[5m])` and `quantile(0.9, metric[5m])`, we need 2 entries in inference config`.
+
+Moreover, if we now get `quantile(0.6, metric[5m])` or `quantile(0.5, metric[10m])`, those queries get punted even if we can support them.
+
+The goal: let the engine understand what a query *needs* and find an existing aggregation that can
+*provide* it, without requiring a one-to-one mapping in config.
+
+---
+
+## Architecture Investigation
+
+Before designing anything, the existing query routing path was traced:
+
+1. An incoming query (PromQL or SQL) is parsed and reduced to a `QueryExecutionContext`.
+2. Inside that process, `find_query_config` (exact string match) or `find_query_config_sql`
+   (structural AST match) look up a `QueryConfig` in `InferenceConfig.query_configs`.
+3. `QueryConfig` is nothing more than a join record: query string → list of `aggregation_id`s.
+4. `get_aggregation_id_info` then looks up those IDs in `StreamingConfig` to get the actual
+   `AggregationConfig` (sketch type, window, labels, etc.).
+
+The key insight: **all capability information lives in `AggregationConfig` inside `StreamingConfig`**.
+The `QueryConfig` table is just indirection that requires manual pre-population. The fix is to
+skip it and match against `AggregationConfig` directly when no pre-configured entry exists.
+
+So the logic now is (a) define what the query needs (QueryRequirements), (b) match QueryRequirements against all the available aggregation_ids to see if there is a match. There is no explicit CapabilityProvidedBySketch data structure
+
+---
+
+## Design Questions and Answers
+
+The following questions were worked through before writing a single line of implementation.
+
+### Q1: When multiple aggregations are compatible, which one wins?
+**Decision**: Prefer the largest `window_size`. Encapsulated in a separate `aggregation_priority`
+comparator function so this policy is swappable later without touching the matching logic.
+
+### Q2: Label compatibility — how strict?
+**Decision**: Strict exact match for now. A sketch grouped by `{job, instance}` does **not** serve
+a query that groups by `{job}` only, even though collapsing labels is mathematically valid for
+simple accumulators (Sum, Min, Max). The reason: for sketch types (KLL, CountMin), label collapsing
+is not well-defined. Adding a TODO to relax this to "superset ok" for simple accumulators in a
+future iteration.
+
+### Q3: Spatial filter compatibility?
+**Decision**: If the stored aggregation has a non-empty `spatial_filter` and the query's normalized
+filter differs (or is absent), reject. Never silently serve data filtered to `{env="prod"}` to a
+query that expects unfiltered data.
+
+### Q4: Multi-population sketches (SetAggregator / DeltaSetAggregator)?
+These require two aggregation IDs: one "value" aggregation and one "key" aggregation. The existing
+`get_aggregation_id_info` already distinguishes them by type: `SetAggregator` and
+`DeltaSetAggregator` are key aggregations; everything else is a value aggregation.
+
+**Decision**: Capability matching finds the value aggregation first (based on the statistic). If
+the matched value type is a "multi-population" type (`MultipleSumAccumulator`,
+`MultipleMinMaxAccumulator`, `MultipleIncreaseAccumulator`, `CountMinSketchWithHeap`), the matcher
+then separately searches for a key aggregation (`SetAggregator` or `DeltaSetAggregator`) on the
+same metric. Both IDs are required; if either is missing, the match fails.
+
+### Q5: Backward compatibility — keep old `query_configs` path?
+**Decision**: Yes, as a primary route. The `query_configs` lookup runs first; capability matching
+fires only when no pre-configured entry is found. This means existing deployments change behavior
+only for queries that had no config entry. A `warn!` log is emitted whenever capability matching
+is used, so operators can detect fallback usage.
+
+### Q6: Rich error messages on no-match?
+**Decision**: Deferred. Collecting per-candidate rejection reasons adds significant complexity.
+The matcher returns `None` on failure for now.
+
+### Q7: How to model `avg` (needs both Sum and Count)?
+**Decision**: `QueryRequirements` holds `Vec<Statistic>`. For `avg`, this is `[Sum, Count]`.
+All statistics in the vec must be satisfied by aggregations that share the **same** `window_size`
+and `grouping_labels`. This ensures temporal consistency.
+
+### Q8: How is window type (sliding vs tumbling) expressed in requirements?
+Framing the requirement as a specific `window_type` was considered but rejected. Instead,
+`QueryRequirements` stores only `data_range_ms: Option<u64>` — the span of historical data the
+query reads. Both tumbling and sliding aggregations can satisfy this, subject to different
+compatibility rules:
+
+- **Tumbling**: `data_range_ms` must be a positive integer multiple of `window_size_ms` (so
+  multiple buckets can be merged to cover the range).
+- **Sliding**: `data_range_ms` must equal `window_size_ms` exactly (a sliding window precomputes
+  exactly one range per timestamp; you can't merge overlapping windows).
+- **Spatial-only** (`data_range_ms = None`): any window is compatible.
+
+### Q9: Where does the capability matching logic live?
+**Decision**: `sketch_db_common` (the shared crate). Rationale: this logic is pure — it takes a
+map of `AggregationConfig` values and a `QueryRequirements` and produces an `AggregationIdInfo`.
+It has no dependency on query engine internals. Putting it in common means the planner and other
+components can eventually reuse it.
+
+`AggregationIdInfo` (previously defined in `simple_engine.rs`) was moved to `sketch_db_common` as
+a prerequisite, since the common function needs to return it.
+
+`StreamingConfig` (in `asap-query-engine`) gets a thin wrapper method that delegates to the
+common function, so call sites inside the engine don't need to reach into common directly.
+
+### Q10: `aggregation_sub_type` — does it matter for matching?
+**Decision**: Yes. `Min` requires `aggregation_sub_type == "min"`, `Max` requires `"max"`. The
+`required_sub_type(statistic)` helper encodes this. Other statistics have no sub-type constraint.
+
+### Q11: For `Vec<Statistic>` — must all statistics agree on window and labels?
+**Decision**: Yes. For `avg = [Sum, Count]`, the matched Sum aggregation and the matched Count
+aggregation must have the same `window_size` and `grouping_labels`. This is the simpler, safer
+choice — mixing aggregations with different windows or label granularities would produce
+semantically incorrect results.
+
+---
+
+## What Was Rejected
+
+### "Translate PromQL to SQL and execute via DataFusion SQL engine"
+Considered as a broader architectural direction. Rejected for this feature because:
+- Data is stored as binary sketches (KLL, CountMin, etc.), not raw values. SQL aggregation
+  functions cannot merge sketches natively.
+- Every sketch operation would need a DataFusion UDF, recreating the existing operator logic
+  with more indirection.
+- The existing `execute_plan()` path already uses DataFusion as an execution *framework* with
+  custom physical operators — that is the right abstraction boundary, not SQL strings.
+
+### Merging PromQL and SQL `build_query_execution_context` paths
+The two build paths (PromQL and SQL) were kept separate. They parse different syntaxes into
+different AST types. Merging them would require a common intermediate representation before the
+current `QueryExecutionContext` and would not reduce complexity. The shared logic is the capability
+matching layer, not the parsing layer.
+
+### Rich rejection errors
+Collecting per-candidate rejection reasons (e.g. "found KLL for metric X but window 15 m doesn't
+match 5 m query") was considered. Deferred: the matching logic touches every candidate and
+collecting structured reasons multiplies the implementation surface significantly. Simple `None`
+return with `debug!` logging is sufficient for now.
+
+---
+
+## Final Architecture
+
+```
+Incoming query (PromQL or SQL)
+        │
+        ▼
+   Parse query AST
+        │
+        ▼
+Try find_query_config / find_query_config_sql    ← existing path (unchanged)
+        │
+        ├── found ──► get_aggregation_id_info(config) ──► AggregationIdInfo
+        │
+        └── not found ──► warn!("falling back to capability matching")
+                              │
+                              ▼
+                     build_query_requirements_{promql|sql}
+                     → QueryRequirements {
+                           metric, statistics: Vec<Statistic>,
+                           data_range_ms, grouping_labels,
+                           spatial_filter_normalized
+                       }
+                              │
+                              ▼
+                     StreamingConfig::find_compatible_aggregation(&requirements)
+                     → sketch_db_common::find_compatible_aggregation(
+                           &self.aggregation_configs, requirements
+                       )
+                     → Option<AggregationIdInfo>
+```
+
+The `find_compatible_aggregation` function in `sketch_db_common`:
+1. For each statistic, collects candidates from `StreamingConfig` passing all filters
+   (metric, type, sub-type, window, labels, spatial filter).
+2. Sorts candidates by `aggregation_priority` (largest window first).
+3. For `Vec<Statistic>`, ensures all statistics are satisfied by configs agreeing on
+   window and labels.
+4. If the value aggregation type is multi-population, also finds the paired key aggregation.
+5. Returns `AggregationIdInfo` or `None`.