Use generic way to parallelize output for file()

+ disable parallelization for storage Null

src/Storages/StorageFile.cpp

@@ -791,15 +791,7 @@ Pipe StorageFile::read(
             std::move(read_buffer)));
     }
 
-    Pipe pipe = Pipe::unitePipes(std::move(pipes));
-    /// Parallelize output as much as possible
-    /// Note: number of streams can be 0 if paths is empty
-    ///       It happens if globs in file(path, ...) expands to empty set i.e. no files to process
-    if (num_streams > 0 && num_streams < max_num_streams)
-    {
-        pipe.resize(max_num_streams);
-    }
-    return pipe;
+    return Pipe::unitePipes(std::move(pipes));
 }
 
 

src/Storages/StorageNull.h

@@ -42,6 +42,8 @@ public:
             std::make_shared<NullSource>(storage_snapshot->getSampleBlockForColumns(column_names)));
     }
 
+    bool parallelizeOutputAfterReading() const override { return false; }
+
     bool supportsParallelInsert() const override { return true; }
 
     SinkToStoragePtr write(const ASTPtr &, const StorageMetadataPtr & metadata_snapshot, ContextPtr) override