DuckDB 是近年来颇受关注的OLAP数据库，号称是OLAP领域的SQLite，以精巧简单，性能优异而著称。笔者前段时间在调研Doris的Pipeline的算子并行方案，而DuckDB基于论文《Morsel-Driven Parallelism: A NUMA-Aware Query Evaluation Framework for the Many-Core Age》实现SQL算子的高效并行化的Pipeline执行引擎，所以笔者花了一些时间进行了学习和总结，这里结合了Mark Raasveldt进行的分享和原始代码来一一剖析DuckDB在执行算子并行上的具体实现。

1. 基础知识

问题1：并行task的数目由什么决定 ？

Pipeline的核心是：Morsel-Driven，数据是拆分成了小部分的数据。所以并行Task的核心是：能够利用多线程来处理数据，每一个数据拆分为小部分，所以拆分并行的数目由Source决定。

DuckDB在GlobalSource上实现了一个虚函数MaxThread来决定task数目：

每一个算子的GlobalSource抽象了自己的并行度：

问题2：并行task的怎么样进行多线程同步：

• 多线程的竞争只会发生在SinkOperator上，也就是Pipeline的尾端。
• parallelism-aware的算法需要实现在Sink端
• 其他的非Sink operators (比如：Hash Join Probe, Projection, Filter等)， 不需要感知多线程同步的问题

问题3：DuckDB的是如何抽象接口的：

Sink的Opeartor 定义了两种类型：GlobalState, LocalState

1. GlobalState: 每个查询的Operator全局只有一个GlobalSinkState，记录全局部分的信息

class PhysicalOperator {
public:
	unique_ptr<GlobalSinkState> sink_state;

1. LocalState: 每个查询的PipelineExecutor都有一个LocalSinkState，都是局部私有

//! The Pipeline class represents an execution pipeline
class PipelineExecutor {
private:
	//! The local sink state (if any)
	unique_ptr<LocalSinkState> local_sink_state;

后续会详细解析不同的sink之间的LocalState和GlobalState如何配合的，核心部分如下：

Sink ：处理LocalState的数据

Combine：合并LocalState到GlobalState之中

2. 核心算子的并行

这部分进行各个算子的源码剖析，笔者在源码的关键部分加上了中文注释，以方便大家的理解

Sort算子

• Sink接口：这里需要注意的是DuckDB排序是进行了列转行的工作的，后续读取时需要行转列。Sink这部分相当于实现了部分数据的排序工作。

SinkResultType PhysicalOrder::Sink(ExecutionContext &context, GlobalSinkState &gstate_p, LocalSinkState &lstate_p,
                                   DataChunk &input) const {
	auto &lstate = (OrderLocalSinkState &)lstate_p;
        
      // keys 是排序的列block，payload是输出的排序后数据，这里调用LocalState的SinkChunk，进行数据的转行，
	local_sort_state.SinkChunk(keys, payload);

	// 数据达到内存阈值的时候进行基数排序处理，排序之后的结果存入LocalState的本地的SortedBlock中
	if (local_sort_state.SizeInBytes() >= gstate.memory_per_thread) {
		local_sort_state.Sort(global_sort_state, true);
	}
	return SinkResultType::NEED_MORE_INPUT;
}

• Combine接口： 加锁，拷贝sorted block到Global State

void PhysicalOrder::Combine(ExecutionContext &context, GlobalSinkState &gstate_p, LocalSinkState &lstate_p) const {
	auto &gstate = (OrderGlobalSinkState &)gstate_p;
	auto &lstate = (OrderLocalSinkState &)lstate_p;
        // 排序剩余内存中不满的数据
	local_sort_state.Sort(*this, external || !local_sort_state.sorted_blocks.empty());

	// Append local state sorted data to this global state
	lock_guard<mutex> append_guard(lock);
	for (auto &sb : local_sort_state.sorted_blocks) {
		sorted_blocks.push_back(move(sb));
	}
}

• MergeTask：启动核数相同的task来进行Merge (这里可以看出DuckDB对于多线程的使用是很激进的)， 这里是通过Event的机制实现的

void Schedule() override {
		auto &context = pipeline->GetClientContext();
		idx_t num_threads = ts.NumberOfThreads();

		vector<unique_ptr<Task>> merge_tasks;
		for (idx_t tnum = 0; tnum < num_threads; tnum++) {
			merge_tasks.push_back(make_unique<PhysicalOrderMergeTask>(shared_from_this(), context, gstate));
		}
		SetTasks(move(merge_tasks));
	}

class PhysicalOrderMergeTask : public ExecutorTask {
public:
	TaskExecutionResult ExecuteTask(TaskExecutionMode mode) override {
		// Initialize merge sorted and iterate until done
		auto &global_sort_state = state.global_sort_state;
		MergeSorter merge_sorter(global_sort_state, BufferManager::GetBufferManager(context));
		
        // 加锁，获取两路，不断进行两路归并，最终完成全局排序。
	while (true) {
		{
			lock_guard<mutex> pair_guard(state.lock);
			if (state.pair_idx == state.num_pairs) {
				break;
			}
			GetNextPartition();
		}
		MergePartition();
	}
		event->FinishTask();
		return TaskExecutionResult::TASK_FINISHED;
	}

聚合算子(这里分析的是Prefetch Agg Operator算子)

• Sink接口：和Sort算子一样，这里拆分为Group Chunk和Aggregate Input Chunk，可以理解为代表聚合时的key与value列。注意此时Sink接口上的聚合是在LocalSinkState上完成的。

SinkResultType PhysicalPerfectHashAggregate::Sink(ExecutionContext &context, GlobalSinkState &state,
                                                  LocalSinkState &lstate_p, DataChunk &input) const {
	lstate.ht->AddChunk(group_chunk, aggregate_input_chunk);
}


void PerfectAggregateHashTable::AddChunk(DataChunk &groups, DataChunk &payload) {
	auto address_data = FlatVector::GetData<uintptr_t>(addresses);
	memset(address_data, 0, groups.size() * sizeof(uintptr_t));
	D_ASSERT(groups.ColumnCount() == group_minima.size());

	// 计算group key列对应的entry的位置
	idx_t current_shift = total_required_bits;
	for (idx_t i = 0; i < groups.ColumnCount(); i++) {
		current_shift -= required_bits[i];
		ComputeGroupLocation(groups.data[i], group_minima[i], address_data, current_shift, groups.size());
	}

	// 通过data加上面的entry位置 + tuple的偏移量，计算出对应的内存地址，并进行init
	idx_t needs_init = 0;
	for (idx_t i = 0; i < groups.size(); i++) {
		D_ASSERT(address_data[i] < total_groups);
		const auto group = address_data[i];
		address_data[i] = uintptr_t(data) + address_data[i] * tuple_size;
	}
	RowOperations::InitializeStates(layout, addresses, sel, needs_init);

	// after finding the group location we update the aggregates
	idx_t payload_idx = 0;
	auto &aggregates = layout.GetAggregates();
	for (idx_t aggr_idx = 0; aggr_idx < aggregates.size(); aggr_idx++) {
		auto &aggregate = aggregates[aggr_idx];
		auto input_count = (idx_t)aggregate.child_count;
                // 进行聚合的Update操作
		RowOperations::UpdateStates(aggregate, addresses, payload, payload_idx, payload.size());
	}
}

• Combine接口： 加锁，merge local hash table 与 global hash table

void PhysicalPerfectHashAggregate::Combine(ExecutionContext &context, GlobalSinkState &gstate_p,
                                           LocalSinkState &lstate_p) const {
	auto &lstate = (PerfectHashAggregateLocalState &)lstate_p;
	auto &gstate = (PerfectHashAggregateGlobalState &)gstate_p;

	lock_guard<mutex> l(gstate.lock);
	gstate.ht->Combine(*lstate.ht);
}

        // local state的地址vector
	Vector source_addresses(LogicalType::POINTER);
       // global state的地址vector
	Vector target_addresses(LogicalType::POINTER);
	auto source_addresses_ptr = FlatVector::GetData<data_ptr_t>(source_addresses);
	auto target_addresses_ptr = FlatVector::GetData<data_ptr_t>(target_addresses);

	// 遍历所有hash table的表，然后进行合并对应能够合并的key
	data_ptr_t source_ptr = other.data;
	data_ptr_t target_ptr = data;
	idx_t combine_count = 0;
	idx_t reinit_count = 0;
	const auto &reinit_sel = *FlatVector::IncrementalSelectionVector();
	for (idx_t i = 0; i < total_groups; i++) {
		auto has_entry_source = other.group_is_set[i];
		// we only have any work to do if the source has an entry for this group
		if (has_entry_source) {
			auto has_entry_target = group_is_set[i];
			if (has_entry_target) {
				// both source and target have an entry: need to combine
				source_addresses_ptr[combine_count] = source_ptr;
				target_addresses_ptr[combine_count] = target_ptr;
				combine_count++;
				if (combine_count == STANDARD_VECTOR_SIZE) {
					RowOperations::CombineStates(layout, source_addresses, target_addresses, combine_count);
					combine_count = 0;
				}
			} else {
				group_is_set[i] = true;
				// only source has an entry for this group: we can just memcpy it over
				memcpy(target_ptr, source_ptr, tuple_size);
				// we clear this entry in the other HT as we "consume" the entry here
				other.group_is_set[i] = false;
			}
		}
		source_ptr += tuple_size;
		target_ptr += tuple_size;
	}

        // 做对应的merge操作
	RowOperations::CombineStates(layout, source_addresses, target_addresses, combine_count);

Join算子

• Sink接口：和Sort算子一样，注意此时Sink接口上的hash 表是在LocalSinkState上完成的。

SinkResultType PhysicalHashJoin::Sink(ExecutionContext &context, GlobalSinkState &gstate_p, LocalSinkState &lstate_p,
                                      DataChunk &input) const {
	auto &gstate = (HashJoinGlobalSinkState &)gstate_p;
	auto &lstate = (HashJoinLocalSinkState &)lstate_p;

	lstate.join_keys.Reset();
	lstate.build_executor.Execute(input, lstate.join_keys);
	// build the HT
	auto &ht = *lstate.hash_table;
	if (!right_projection_map.empty()) {
		// there is a projection map: fill the build chunk with the projected columns
		lstate.build_chunk.Reset();
		lstate.build_chunk.SetCardinality(input);
		for (idx_t i = 0; i < right_projection_map.size(); i++) {
			lstate.build_chunk.data[i].Reference(input.data[right_projection_map[i]]);
		}
                // 构建local state的hash 表
		ht.Build(lstate.join_keys, lstate.build_chunk)

	return SinkResultType::NEED_MORE_INPUT;
}

• Combine接口： 加锁，拷贝local state的hash表到global state

void PhysicalHashJoin::Combine(ExecutionContext &context, GlobalSinkState &gstate_p, LocalSinkState &lstate_p) const {
	auto &gstate = (HashJoinGlobalSinkState &)gstate_p;
	auto &lstate = (HashJoinLocalSinkState &)lstate_p;
	if (lstate.hash_table) {
		lock_guard<mutex> local_ht_lock(gstate.lock);
		gstate.local_hash_tables.push_back(move(lstate.hash_table));
	}
}

• MergeTask：启动核数相同的task来进行Hash table的Merge (这里可以看出DuckDB对于多线程的使用是很激进的)， 每个任务merge一部分Block(DuckDB之中的行数据，落盘使用）

void Schedule() override {
		auto &context = pipeline->GetClientContext();

		vector<unique_ptr<Task>> finalize_tasks;
		auto &ht = *sink.hash_table;
		const auto &block_collection = ht.GetBlockCollection();
		const auto &blocks = block_collection.blocks;
		const auto num_blocks = blocks.size();
		if (block_collection.count < PARALLEL_CONSTRUCT_THRESHOLD && !context.config.verify_parallelism) {
			// Single-threaded finalize
			finalize_tasks.push_back(
			    make_unique<HashJoinFinalizeTask>(shared_from_this(), context, sink, 0, num_blocks, false));
		} else {
			// Parallel finalize
			idx_t num_threads = TaskScheduler::GetScheduler(context).NumberOfThreads();
			auto blocks_per_thread = MaxValue<idx_t>((num_blocks + num_threads - 1) / num_threads, 1);

			idx_t block_idx = 0;
			for (idx_t thread_idx = 0; thread_idx < num_threads; thread_idx++) {
				auto block_idx_start = block_idx;
				auto block_idx_end = MinValue<idx_t>(block_idx_start + blocks_per_thread, num_blocks);
				finalize_tasks.push_back(make_unique<HashJoinFinalizeTask>(shared_from_this(), context, sink,
				                                                           block_idx_start, block_idx_end, true));
				block_idx = block_idx_end;
				if (block_idx == num_blocks) {
					break;
				}
			}
		}
		SetTasks(move(finalize_tasks));
	}

template <bool PARALLEL>
static inline void InsertHashesLoop(atomic<data_ptr_t> pointers[], const hash_t indices[], const idx_t count,
                                    const data_ptr_t key_locations[], const idx_t pointer_offset) {
	for (idx_t i = 0; i < count; i++) {
		auto index = indices[i];
		if (PARALLEL) {
			data_ptr_t head;
			do {
				head = pointers[index];
				Store<data_ptr_t>(head, key_locations[i] + pointer_offset);
			} while (!std::atomic_compare_exchange_weak(&pointers[index], &head, key_locations[i]));
		} else {
			// set prev in current key to the value (NOTE: this will be nullptr if there is none)
			Store<data_ptr_t>(pointers[index], key_locations[i] + pointer_offset);

			// set pointer to current tuple
			pointers[index] = key_locations[i];
		}
	}
}

• 并行扫描hash表，进行outer数据的处理：

void PhysicalHashJoin::GetData(ExecutionContext &context, DataChunk &chunk, GlobalSourceState &gstate_p,
                               LocalSourceState &lstate_p) const {
	auto &sink = (HashJoinGlobalSinkState &)*sink_state;
	auto &gstate = (HashJoinGlobalSourceState &)gstate_p;
	auto &lstate = (HashJoinLocalSourceState &)lstate_p;
	sink.scanned_data = true;

	if (!sink.external) {
		if (IsRightOuterJoin(join_type)) {
			{
				lock_guard<mutex> guard(gstate.lock);
                                // 拆解扫描部分hash表的数据
				lstate.ScanFullOuter(sink, gstate);
			}
                        // 扫描hash表读取数据
			sink.hash_table->GatherFullOuter(chunk, lstate.addresses, lstate.full_outer_found_entries);
		}
		return;
	}
}


void HashJoinLocalSourceState::ScanFullOuter(HashJoinGlobalSinkState &sink, HashJoinGlobalSourceState &gstate) {
	auto &fo_ss = gstate.full_outer_scan;
	idx_t scan_index_before = fo_ss.scan_index;
	full_outer_found_entries = sink.hash_table->ScanFullOuter(fo_ss, addresses);
	idx_t scanned = fo_ss.scan_index - scan_index_before;
	full_outer_in_progress = scanned;
}

小结

• DuckDB在多线程同步，核心就是在Combine的时候：加锁，并发是通过原子变量的方式实现并发写入hash表的操作
• 通过local/global 拆分私有内存和公共内存，并发的基础是在私有内存上进行运算，同步的部分主要在公有内存的更新

3. Spill To Disk的实现

DuckDB并没有如笔者预期的实现异步IO， 所以任意的执行线程是有可能Stall在系统的I/O调度上的，我想大概率是DuckDB本身的定位对于高并发场景的支持不是那么敏感所导致的。这里他们也作为了后续TODO的计划之一。

4. 参考资料

DuckDB源码

Push-Based Execution in DuckDB