Stage 3: satisfies checks the error metric

crates/logical-optimizer/src/accuracy/estimators/mod.rs

@@ -253,4 +253,7 @@ impl AccuracyModel for EstimatorAccuracy<'_> {
     fn satisfies(&self, guarantee: &ResultGuarantee, target: &AccuracyTarget) -> bool {
         self.base.satisfies(guarantee, target)
     }
+    fn answers(&self, statistic: &SketchStatistic, guarantee: &ResultGuarantee) -> bool {
+        self.base.answers(statistic, guarantee)
+    }
 }

crates/logical-optimizer/src/accuracy/mod.rs

@@ -68,6 +68,34 @@ pub trait AccuracyModel {
     /// Compare the dimensions requested by `target`. Unknown required
     /// dimensions fail; selection separately excludes missing accuracy evidence.
     fn satisfies(&self, guarantee: &ResultGuarantee, target: &AccuracyTarget) -> bool;
+
+    /// Whether `guarantee` bounds the error a target on `statistic` is
+    /// stated in, so that [`Self::satisfies`] compares like with like. An
+    /// exact guarantee answers every statistic; otherwise the metric must be
+    /// one the statistic's ε is measured in. A deployment with a registered
+    /// cross-metric conversion overrides this.
+    fn answers(&self, statistic: &SketchStatistic, guarantee: &ResultGuarantee) -> bool {
+        use ErrorMetric::*;
+        guarantee.is_exact()
+            || match statistic {
+                SketchStatistic::Quantile { .. } => {
+                    matches!(guarantee.metric, Rank | RelativeValue)
+                }
+                SketchStatistic::Cardinality => {
+                    matches!(guarantee.metric, Cardinality | RelativeValue)
+                }
+                SketchStatistic::FrequencyL2 | SketchStatistic::FrequencyEntropy => {
+                    guarantee.metric == RelativeValue
+                }
+                SketchStatistic::PointCount { .. } => {
+                    matches!(guarantee.metric, Frequency | L2Frequency)
+                }
+                // A top-k target bounds the item scores, as Pass 1 checks.
+                SketchStatistic::TopK { .. } => {
+                    matches!(guarantee.metric, Frequency | L2Frequency | TopKMembership)
+                }
+            }
+    }
 }
 
 /// The built-in estimator and composition models, with conservative target checks.
@@ -152,4 +180,24 @@ mod tests {
             DefaultAccuracyModel.satisfies(&ResultGuarantee::exact("x"), &AccuracyTarget::Exact)
         );
     }
+
+    /// A bound in another statistic's metric does not answer a target:
+    /// Count-Min's L1 frequency bound says nothing about a distinct count.
+    #[test]
+    fn answers_requires_the_statistics_metric() {
+        let frequency = ResultGuarantee {
+            metric: ErrorMetric::Frequency,
+            ..abs(0.01, 0.01)
+        };
+        let count = SketchStatistic::PointCount {
+            key: asap_types::ir::scalar::ColumnRef::SampleValue,
+            value: None,
+        };
+        assert!(DefaultAccuracyModel.answers(&count, &frequency));
+        assert!(!DefaultAccuracyModel.answers(&SketchStatistic::Cardinality, &frequency));
+        assert!(!DefaultAccuracyModel.answers(&SketchStatistic::Quantile { q: 0.5 }, &frequency));
+        assert!(!DefaultAccuracyModel.answers(&SketchStatistic::FrequencyL2, &abs(0.01, 0.01)));
+        assert!(DefaultAccuracyModel
+            .answers(&SketchStatistic::Cardinality, &ResultGuarantee::exact("x")));
+    }
 }

crates/plan-selection/src/lib.rs

@@ -1275,6 +1275,12 @@ fn build_violation(
     let Some(guarantee) = models.accuracy.local_guarantee(family, statistic) else {
         return Some(format!("no accuracy model for {name}; target {target:?}"));
     };
+    if !models.accuracy.answers(statistic, &guarantee) {
+        return Some(format!(
+            "{name} guarantees a {:?} bound, which does not bound {statistic:?}; target {target:?}",
+            guarantee.metric
+        ));
+    }
     if !models.accuracy.satisfies(&guarantee, target) {
         return Some(format!(
             "{name} guarantees bound {:?}, failure probability {:?}, which misses target \
@@ -2520,6 +2526,33 @@ mod tests {
         }
     }
 
+    /// A guarantee in another statistic's metric does not satisfy a target:
+    /// CountSketch's L2 frequency bound answers the top-k scores it was
+    /// built for, but not a distinct count, however loose the target.
+    #[test]
+    fn a_bound_in_another_metric_misses_the_target() {
+        let candidates = candidates();
+        let build = OperatorNode::reachable(&candidates[2].roots[0])
+            .into_iter()
+            .find(|node| {
+                matches!(&node.operator, Operator::ASAP(ASAPOp::SummaryAgg { family: FieldDataType::Sketch(kind, _), .. })
+                    if *kind.algorithm() == SketchAlgorithm::CountSketchWithHeap)
+            })
+            .expect("a CountSketch build");
+        let loose = AccuracyTarget::EpsilonDelta {
+            epsilon: 0.5,
+            delta: 0.5,
+        };
+        let models = PlanningModels::builtin();
+        assert_eq!(
+            build_violation(&build, &SketchStatistic::TopK { k: 10 }, &loose, &models),
+            None
+        );
+        let reason = build_violation(&build, &SketchStatistic::Cardinality, &loose, &models)
+            .expect("an L2 frequency bound does not bound a distinct count");
+        assert!(reason.contains("does not bound Cardinality"), "{reason}");
+    }
+
     /// A candidate that cannot be checked is rejected with its reason; the
     /// others are still selected among.
     #[test]