perf(cache): use the quick_cache backend for the session metadata cache

rust/lance-core/src/cache/quick.rs

@@ -3,8 +3,8 @@
 
 //! [`CacheBackend`] backed by [quick_cache](https://crates.io/crates/quick_cache),
 //! whose hit path is one atomic bit — no read-op channel or inline
-//! housekeeping. Used for the session index cache, which sees thousands of
-//! cache reads per query.
+//! housekeeping. Used for the session index and metadata caches; the index
+//! cache sees thousands of cache reads per query.
 
 use std::pin::Pin;
 

rust/lance/src/session.rs

@@ -97,7 +97,8 @@ impl Session {
     ///
     /// - ***index_cache_size***: the size of the index cache, backed by
     ///   [`QuickCacheBackend`].
-    /// - ***metadata_cache_size***: the size of the metadata cache.
+    /// - ***metadata_cache_size***: the size of the metadata cache, backed by
+    ///   [`QuickCacheBackend`].
     /// - ***store_registry***: the object store registry to use when opening
     ///   datasets. This determines which schemes are available, and also allows
     ///   re-using object stores.
@@ -110,7 +111,9 @@ impl Session {
             index_cache: GlobalIndexCache(LanceCache::with_backend(Arc::new(
                 QuickCacheBackend::with_capacity(index_cache_size),
             ))),
-            metadata_cache: GlobalMetadataCache(LanceCache::with_capacity(metadata_cache_size)),
+            metadata_cache: GlobalMetadataCache(LanceCache::with_backend(Arc::new(
+                QuickCacheBackend::with_capacity(metadata_cache_size),
+            ))),
             index_extensions: HashMap::new(),
             store_registry,
             spill_store: Arc::new(LocalSpillStore::default()),
@@ -120,15 +123,17 @@ impl Session {
     /// Create a session with a custom index cache backend.
     ///
     /// The provided backend will be used for caching index data. The metadata
-    /// cache will use the default Moka-based backend with the given capacity.
+    /// cache uses a [`QuickCacheBackend`] with the given capacity.
     pub fn with_index_cache_backend(
         index_cache_backend: Arc<dyn CacheBackend>,
         metadata_cache_size: usize,
         store_registry: Arc<ObjectStoreRegistry>,
     ) -> Self {
         Self {
             index_cache: GlobalIndexCache(LanceCache::with_backend(index_cache_backend)),
-            metadata_cache: GlobalMetadataCache(LanceCache::with_capacity(metadata_cache_size)),
+            metadata_cache: GlobalMetadataCache(LanceCache::with_backend(Arc::new(
+                QuickCacheBackend::with_capacity(metadata_cache_size),
+            ))),
             index_extensions: HashMap::new(),
             store_registry,
             spill_store: Arc::new(LocalSpillStore::default()),