Clarify vector_usearch cache-only behavior

Author: anoop-narang

src/lib.rs

@@ -100,6 +100,7 @@ use datafusion::prelude::SessionContext;
 /// - `cosine_distance(col, query)`      — cosine distance
 /// - `negative_dot_product(col, query)` — negated inner product
 /// - `vector_usearch(table, query, k)`  — explicit ANN table function
+///   (cache-only for async-backed resolvers; does not trigger async loads)
 /// - [`USearchRule`]                    — optimizer rewrite rule
 ///
 /// The [`USearchQueryPlanner`] must be installed at `SessionState` build time
@@ -110,6 +111,8 @@ pub fn register_all(ctx: &SessionContext, registry: Arc<dyn VectorIndexResolver>
     ctx.register_udf(ScalarUDF::new_from_impl(negative_dot_product_udf()));
     ctx.register_udtf(
         "vector_usearch",
+        // `vector_usearch()` is synchronous and therefore cache-only for
+        // async-backed resolvers.
         Arc::new(USearchUDTF::new(registry.clone())),
     );
     ctx.add_optimizer_rule(Arc::new(USearchRule::new(registry)));

src/udtf.rs

@@ -41,6 +41,11 @@ use crate::registry::VectorIndexResolver;
 ///
 /// Returns `(key: UInt64, _distance: Float32)`. Join with your data table on
 /// the key column to retrieve full rows.
+///
+/// This entry point is synchronous. For async-backed [`VectorIndexResolver`]
+/// implementations, it only works when the target index is already loaded in
+/// the local cache. `vector_usearch()` does not call `ensure_loaded()` and
+/// cannot trigger async index loads.
 pub struct USearchUDTF {
     registry: Arc<dyn VectorIndexResolver>,
 }
@@ -87,7 +92,9 @@ impl TableFunctionImpl for USearchUDTF {
 
         let registered = self.registry.resolve(&table_name).ok_or_else(|| {
             DataFusionError::Execution(format!(
-                "vector_usearch: table '{table_name}' not registered"
+                "vector_usearch: table '{table_name}' is not loaded locally. \
+This synchronous path only checks the local cache and cannot trigger async \
+loads. Use the optimizer/planner vector query path or pre-load the index first."
             ))
         })?;