quack: refuse an empty group universe in lower_group_avg; record lancedb remote cost

.claude/board/entries/2026-09-25-lance12-lancedb039-sweep.md

@@ -5,7 +5,7 @@
 ## What moved
 - `lance*` `11.*` → `12.*` and `lancedb` `0.38.*` → `0.39.*`, measured against crates.io. lancedb 0.39.0 pins `lance = "=12.0.0"`. arrow ^58 and datafusion ^54 are unmoved.
 - **`object_store` 0.13 → 0.14 for our direct dependency.** lance 12 requires ^0.14.1, while datafusion 54 stays on ^0.13.2, so both majors are in the graph (upstream's split). Every direct call site here hands the store or a `Path` to lance or to lance-graph-hydrate. On 0.13, `memwal_atomicity_probe` and `hydration_probe` failed with E0308 ("multiple different versions of crate `object_store`").
-- **lancedb `remote` on the consumers behind `lancedb-sdk`** (lance-graph, surreal_container). 0.39.0 does not compile without it: `pub mod job;` is ungated while `job.rs` uses the `remote`-gated `Error::Http`. It is set at crate level, so it is active only with the optional dependency.
+- **lancedb `remote` on the consumers behind `lancedb-sdk`** (lance-graph, surreal_container). 0.39.0 does not compile without it: `pub mod job;` is ungated while `job.rs` uses the `remote`-gated `Error::Http`. It is set at crate level, so it is active only with the optional dependency. Measured cost: +10 crates (630 vs 620). reqwest, tonic, prost and http were already present via lance; the delta is arrow-flight, axum 0.7 and 0.8 (two majors), axum-core ×2, matchit ×2, tower-http, serde_path_to_error and httpdate.
 - Sub-crate toolchains (`reader-lm`, `bge-m3`, `python`) 1.95.0 → 1.98.1, matching the root. Builder images for `cognitive-stack`, `symbiont` and `thinking-engine` (was 1.82) → `rust:1.98`.
 
 ## Gates

crates/lance-graph-quack/src/lib.rs

@@ -1244,13 +1244,18 @@ pub struct GroupAvgPlan {
 ///
 /// # Errors
 ///
-/// As [`lower`].
+/// [`LowerError::EmptyGroupUniverse`] when `groups == 0`, for every key kind:
+/// the count half is a `GroupReduce`, whose sink must be a non-empty
+/// `Out::I64`, so a K = 0 plan could never execute. Otherwise as [`lower`].
 pub fn lower_group_avg(
     filter: &Filter,
     key: GroupAddr,
     val: Col,
     groups: u32,
 ) -> Result<GroupAvgPlan, LowerError> {
+    if groups == 0 {
+        return Err(LowerError::EmptyGroupUniverse);
+    }
     let sum_agg = match key {
         GroupAddr::Local(k) => Agg::GroupSumI32 { key: k, val },
         GroupAddr::Via { fk, key } => Agg::GroupSumViaI32 { fk, key, val },
@@ -2131,6 +2136,30 @@ mod tests {
 
     /// FAILS IF a zero-group HAVING lowers to programs the executor would
     /// refuse, instead of being refused at lowering with a named error.
+    #[test]
+    fn lower_group_avg_refuses_an_empty_group_universe_for_every_key_kind() {
+        // The count half is `GroupReduce Count` for every key kind, and that
+        // terminal needs a non-empty `Out::I64`: a K = 0 plan could never run.
+        let filter = Filter::cmp(Col(0), Cmp::EqU32(1));
+        let keys = [
+            GroupAddr::Local(Col(1)),
+            GroupAddr::Pair {
+                hi: Col(1),
+                lo: Col(2),
+                stride: 3,
+            },
+        ];
+        for key in keys {
+            assert_eq!(
+                lower_group_avg(&filter, key, Col(3), 0).map(|_| ()),
+                Err(LowerError::EmptyGroupUniverse),
+                "{key:?}"
+            );
+            // One group lowers: the refusal is about K alone.
+            assert!(lower_group_avg(&filter, key, Col(3), 1).is_ok(), "{key:?}");
+        }
+    }
+
     #[test]
     fn lower_group_having_refuses_an_empty_group_universe() {
         let q = GroupHaving {

crates/lance-graph/Cargo.toml

@@ -57,6 +57,11 @@ lance-namespace = "12.*"
 # (E0599). Same upstream defect as 0.38.0; no 0.39 patch release fixes it.
 # Declared here, not in the workspace entry, so it is active only when
 # `lancedb-sdk` turns the optional dep on. Drop it once upstream gates `job`.
+# Cost, measured with `cargo tree -p lance-graph --features lancedb-sdk`
+# (2026-09-25): +10 crates (630 vs 620). reqwest / tonic / prost / http are
+# already in the graph via lance; the delta is arrow-flight, axum 0.7 + 0.8
+# (two majors), axum-core x2, matchit x2, tower-http, serde_path_to_error,
+# httpdate.
 lancedb = { workspace = true, optional = true, features = ["remote"] }
 nom = "7.1"
 serde = { version = "1", features = ["derive"] }