Remove unnecessary RwLock from TableBuffer.schema

Schema is immutable after table creation - no lock needed.
Just use SchemaRef (Arc<Schema>) directly for zero contention.

Author: tonyalaribe

docs/buffered-write-layer.md

@@ -78,7 +78,7 @@ pub struct MemBuffer {
 
 pub struct TableBuffer {
     buckets: DashMap<i64, TimeBucket>,  // bucket_id → TimeBucket
-    schema: RwLock<SchemaRef>,
+    schema: SchemaRef,  // Immutable after creation
     project_id: Arc<str>,
     table_name: Arc<str>,
 }
@@ -248,7 +248,7 @@ Since MemBuffer uses `UnknownPartitioning` (time buckets) and Delta uses file-ba
 |-----------|-----------|------------|
 | `MemBuffer.tables` | DashMap (lock-free reads) | Very low |
 | `TableBuffer.buckets` | DashMap (lock-free reads) | Very low |
-| `TableBuffer.schema` | RwLock | Very low (rarely changes) |
+| `TableBuffer.schema` | None (immutable `Arc<Schema>`) | None |
 | `TimeBucket.batches` | RwLock | Low (read-heavy workload) |
 
 **Key insight:** Query path uses read locks only. Write path acquires write lock briefly per bucket. Handle caching (`Arc<TableBuffer>`) further reduces contention by avoiding repeated table lookups.

src/mem_buffer.rs

@@ -117,7 +117,7 @@ pub struct MemBuffer {
 
 pub struct TableBuffer {
     buckets: DashMap<i64, TimeBucket>,
-    schema: RwLock<SchemaRef>,
+    schema: SchemaRef,  // Immutable after creation - no lock needed
     project_id: Arc<str>,
     table_name: Arc<str>,
 }
@@ -711,14 +711,14 @@ impl TableBuffer {
     fn new(schema: SchemaRef, project_id: Arc<str>, table_name: Arc<str>) -> Self {
         Self {
             buckets: DashMap::new(),
-            schema: RwLock::new(schema),
+            schema,
             project_id,
             table_name,
         }
     }
 
     pub fn schema(&self) -> SchemaRef {
-        self.schema.read().unwrap().clone()
+        self.schema.clone()  // Arc clone is cheap
     }
 
     /// Insert a batch into this table's appropriate time bucket.