optimize timeful table lookup in GC

crates/re_arrow_store/src/store_gc.rs

@@ -205,8 +205,6 @@ impl DataStore {
     /// Returns the list of `RowId`s that were purged from the store.
     //
     // TODO(jleibs): There are some easy optimizations here if we find GC taking too long:
-    //  - If we stored the entity_path_hash along with timepoints in the metadata_registry we could jump
-    //    directly to the relevant tables instead of needing to iterate over all tables.
     //  - If we know we are clearing almost everything, then we can batch-clear the rows from the
     //    the tables instead of needing to iterate over every single row incrementally.
     fn gc_drop_at_least_num_bytes(
@@ -224,22 +222,22 @@ impl DataStore {
         // 2. Find all tables that potentially hold data associated with that `RowId`
         // 3. Drop the associated row and account for the space we got back
 
-        for (row_id, (timepoint, entity_path_hash)) in &self.metadata_registry.registry {
+        for (&row_id, (timepoint, entity_path_hash)) in &self.metadata_registry.registry {
             if num_bytes_to_drop <= 0.0 {
                 break;
             }
 
-            if protected_rows.contains(row_id) {
+            if protected_rows.contains(&row_id) {
                 continue;
             }
 
             let mut diff: Option<StoreDiff> = None;
 
             // find all tables that could possibly contain this `RowId`
-            for ((timeline, _), table) in &mut self.tables {
-                if let Some(time) = timepoint.get(timeline) {
+            for (&timeline, &time) in timepoint {
+                if let Some(table) = self.tables.get_mut(&(timeline, *entity_path_hash)) {
                     let (removed, num_bytes_removed) =
-                        table.try_drop_row(&self.cluster_cell_cache, *row_id, time.as_i64());
+                        table.try_drop_row(&self.cluster_cell_cache, row_id, time.as_i64());
                     if let Some(inner) = diff.as_mut() {
                         if let Some(removed) = removed {
                             diff = inner.union(&removed);
@@ -257,7 +255,7 @@ impl DataStore {
                 for table in self.timeless_tables.values_mut() {
                     // let deleted_comps = deleted.timeless.entry(ent_path.clone()_hash).or_default();
                     let (removed, num_bytes_removed) =
-                        table.try_drop_row(&self.cluster_cell_cache, *row_id);
+                        table.try_drop_row(&self.cluster_cell_cache, row_id);
                     if let Some(inner) = diff.as_mut() {
                         if let Some(removed) = removed {
                             diff = inner.union(&removed);