[Opt](Variant) merge schema in sync_rowsets to prevents from CPU overhead each time describe table

be/src/cloud/cloud_tablet.cpp

@@ -108,6 +108,36 @@ Status CloudTablet::capture_rs_readers(const Version& spec_version,
     return capture_rs_readers_unlocked(version_path, rs_splits);
 }
 
+Status CloudTablet::merge_rowsets_schema() {
+    // Find the rowset with the max version
+    auto max_version_rowset =
+            std::max_element(
+                    _rs_version_map.begin(), _rs_version_map.end(),
+                    [](const auto& a, const auto& b) {
+                        return !a.second->tablet_schema()
+                                       ? true
+                                       : (!b.second->tablet_schema()
+                                                  ? false
+                                                  : a.second->tablet_schema()->schema_version() <
+                                                            b.second->tablet_schema()
+                                                                    ->schema_version());
+                    })
+                    ->second;
+    TabletSchemaSPtr max_version_schema = max_version_rowset->tablet_schema();
+    // If the schema has variant columns, perform a merge to create a wide tablet schema
+    if (max_version_schema->num_variant_columns() > 0) {
+        std::vector<TabletSchemaSPtr> schemas;
+        std::transform(_rs_version_map.begin(), _rs_version_map.end(), std::back_inserter(schemas),
+                       [](const auto& rs_meta) { return rs_meta.second->tablet_schema(); });
+        // Merge the collected schemas to obtain the least common schema
+        RETURN_IF_ERROR(vectorized::schema_util::get_least_common_schema(schemas, nullptr,
+                                                                         max_version_schema));
+        VLOG_DEBUG << "dump schema: " << max_version_schema->dump_full_schema();
+        _merged_tablet_schema = max_version_schema;
+    }
+    return Status::OK();
+}
+
 // There are only two tablet_states RUNNING and NOT_READY in cloud mode
 // This function will erase the tablet from `CloudTabletMgr` when it can't find this tablet in MS.
 Status CloudTablet::sync_rowsets(int64_t query_version, bool warmup_delta_data) {
@@ -133,6 +163,10 @@ Status CloudTablet::sync_rowsets(int64_t query_version, bool warmup_delta_data)
     if (st.is<ErrorCode::NOT_FOUND>()) {
         clear_cache();
     }
+
+    // Merge all rowset schemas within a CloudTablet
+    RETURN_IF_ERROR(merge_rowsets_schema());
+
     return st;
 }
 
@@ -188,16 +222,7 @@ Status CloudTablet::sync_if_not_running() {
 }
 
 TabletSchemaSPtr CloudTablet::merged_tablet_schema() const {
-    std::shared_lock rdlock(_meta_lock);
-    TabletSchemaSPtr target_schema;
-    std::vector<TabletSchemaSPtr> schemas;
-    for (const auto& [_, rowset] : _rs_version_map) {
-        schemas.push_back(rowset->tablet_schema());
-    }
-    // get the max version schema and merge all schema
-    static_cast<void>(
-            vectorized::schema_util::get_least_common_schema(schemas, nullptr, target_schema));
-    return target_schema;
+    return _merged_tablet_schema;
 }
 
 void CloudTablet::add_rowsets(std::vector<RowsetSharedPtr> to_add, bool version_overlap,

be/src/cloud/cloud_tablet.h

@@ -208,6 +208,9 @@ class CloudTablet final : public BaseTablet {
 
     Status sync_if_not_running();
 
+    // Merge all rowset schemas within a CloudTablet
+    Status merge_rowsets_schema();
+
     CloudStorageEngine& _engine;
 
     // this mutex MUST ONLY be used when sync meta
@@ -246,6 +249,9 @@ class CloudTablet final : public BaseTablet {
     std::mutex _base_compaction_lock;
     std::mutex _cumulative_compaction_lock;
     mutable std::mutex _rowset_update_lock;
+
+    // Schema will be merged from all rowsets when sync_rowsets
+    TabletSchemaSPtr _merged_tablet_schema;
 };
 
 using CloudTabletSPtr = std::shared_ptr<CloudTablet>;

be/src/service/internal_service.cpp

@@ -1159,7 +1159,10 @@ void PInternalService::fetch_remote_tablet_schema(google::protobuf::RpcControlle
                         LOG(WARNING) << "tablet does not exist, tablet id is " << tablet_id;
                         continue;
                     }
-                    tablet_schemas.push_back(res.value()->merged_tablet_schema());
+                    auto schema = res.value()->merged_tablet_schema();
+                    if (schema != nullptr) {
+                        tablet_schemas.push_back(schema);
+                    }
                 }
                 if (!tablet_schemas.empty()) {
                     // merge all