fix order of ranges in MergeTreeDataSelectExecutor

dbms/src/Storages/MergeTree/MergeTreeBlockReadUtils.cpp

@@ -73,7 +73,10 @@ MergeTreeReadTask::MergeTreeReadTask(
     : data_part{data_part_}, mark_ranges{mark_ranges_}, part_index_in_query{part_index_in_query_},
     ordered_names{ordered_names_}, column_name_set{column_name_set_}, columns{columns_}, pre_columns{pre_columns_},
     remove_prewhere_column{remove_prewhere_column_}, should_reorder{should_reorder_}, size_predictor{std::move(size_predictor_)}
-{}
+{
+    /// We need to save marks in reverse order.
+    std::reverse(mark_ranges.begin(), mark_ranges.end());
+}
 
 MergeTreeReadTask::~MergeTreeReadTask() = default;
 

dbms/src/Storages/MergeTree/MergeTreeDataSelectExecutor.cpp

@@ -718,9 +718,6 @@ Pipes MergeTreeDataSelectExecutor::spreadMarkRangesAmongStreams(
     for (size_t i = 0; i < parts.size(); ++i)
     {
         total_rows += parts[i].getRowsCount();
-        /// Let the ranges be listed from right to left so that the leftmost range can be dropped using `pop_back()`.
-        std::reverse(parts[i].ranges.begin(), parts[i].ranges.end());
-
         sum_marks_in_parts[i] = parts[i].getMarksCount();
         sum_marks += sum_marks_in_parts[i];
 
@@ -897,6 +894,8 @@ Pipes MergeTreeDataSelectExecutor::spreadMarkRangesAmongStreamsWithOrder(
                 }
                 new_ranges.emplace_back(range.begin, range.end);
             }
+
+            /// Restore left-to-right order.
             std::reverse(new_ranges.begin(), new_ranges.end());
         }
 

dbms/src/Storages/MergeTree/MergeTreeReadPool.cpp

@@ -28,11 +28,6 @@ MergeTreeReadPool::MergeTreeReadPool(
       column_names{column_names_}, do_not_steal_tasks{do_not_steal_tasks_},
       predict_block_size_bytes{preferred_block_size_bytes_ > 0}, prewhere_info{prewhere_info_}, parts_ranges{parts_}
 {
-    /// reverse from right-to-left to left-to-right
-    /// because 'reverse' was done in MergeTreeDataSelectExecutor
-    for (auto & part_ranges : parts_ranges)
-        std::reverse(std::begin(part_ranges.ranges), std::end(part_ranges.ranges));
-
     /// parts don't contain duplicate MergeTreeDataPart's.
     const auto per_part_sum_marks = fillPerPartInfo(parts_, check_columns_);
     fillPerThreadInfo(threads_, sum_marks_, per_part_sum_marks, parts_, min_marks_for_concurrent_read_);
@@ -79,10 +74,9 @@ MergeTreeReadTaskPtr MergeTreeReadPool::getTask(const size_t min_marks_to_read,
     {
         const auto marks_to_get_from_range = marks_in_part;
 
-        /** Ranges are in right-to-left order, because 'reverse' was done in MergeTreeDataSelectExecutor
+        /// Ranges are in right-to-left order, because 'reverse' was done in 'fillPerThreadInfo'.
-            *     and that order is supported in 'fillPerThreadInfo'.
-            */
         ranges_to_get_from_part = thread_task.ranges;
+        std::reverse(ranges_to_get_from_part.begin(), ranges_to_get_from_part.end());
 
         marks_in_part -= marks_to_get_from_range;
 
@@ -113,11 +107,7 @@ MergeTreeReadTaskPtr MergeTreeReadPool::getTask(const size_t min_marks_to_read,
             marks_in_part -= marks_to_get_from_range;
             need_marks -= marks_to_get_from_range;
         }
-
+        /// Order of ranges was changed to left-to-right due to .pop_back() above.
-        /** Change order to right-to-left, for MergeTreeThreadSelectBlockInputStream to get ranges with .pop_back()
-            *  (order was changed to left-to-right due to .pop_back() above).
-            */
-        std::reverse(std::begin(ranges_to_get_from_part), std::end(ranges_to_get_from_part));
     }
 
     auto curr_task_size_predictor = !per_part_size_predictor[part_idx] ? nullptr
@@ -211,7 +201,6 @@ std::vector<size_t> MergeTreeReadPool::fillPerPartInfo(
 
         /// Read marks for every data part.
         size_t sum_marks = 0;
-        /// Ranges are in right-to-left order, due to 'reverse' in MergeTreeDataSelectExecutor.
         for (const auto & range : part.ranges)
             sum_marks += range.end - range.begin;
 
@@ -251,6 +240,10 @@ void MergeTreeReadPool::fillPerThreadInfo(
 {
     threads_tasks.resize(threads);
 
+    /// Let the ranges be listed from right to left so that the leftmost range can be dropped using `pop_back()`.
+    for (auto & part : parts)
+        std::reverse(part.ranges.begin(), part.ranges.end());
+
     const size_t min_marks_per_thread = (sum_marks - 1) / threads + 1;
 
     for (size_t i = 0; i < threads && !parts.empty(); ++i)

dbms/src/Storages/MergeTree/MergeTreeSelectProcessor.cpp

@@ -95,11 +95,6 @@ try
 
     task_columns = getReadTaskColumns(storage, data_part, required_columns, prewhere_info, check_columns);
 
-    /** @note you could simply swap `reverse` in if and else branches of MergeTreeDataSelectExecutor,
-     * and remove this reverse. */
-    MarkRanges remaining_mark_ranges = all_mark_ranges;
-    std::reverse(remaining_mark_ranges.begin(), remaining_mark_ranges.end());
-
     auto size_predictor = (preferred_block_size_bytes == 0)
         ? nullptr
         : std::make_unique<MergeTreeBlockSizePredictor>(data_part, ordered_names, data_part->storage.getSampleBlock());
@@ -109,7 +104,7 @@ try
     column_name_set = NameSet{column_names.begin(), column_names.end()};
 
     task = std::make_unique<MergeTreeReadTask>(
-        data_part, remaining_mark_ranges, part_index_in_query, ordered_names, column_name_set, task_columns.columns,
+        data_part, all_mark_ranges, part_index_in_query, ordered_names, column_name_set, task_columns.columns,
         task_columns.pre_columns, prewhere_info && prewhere_info->remove_prewhere_column,
         task_columns.should_reorder, std::move(size_predictor));
 

dbms/tests/queries/0_stateless/01078_merge_tree_read_one_thread.reference

@@ -0,0 +1 @@
+300

dbms/tests/queries/0_stateless/01078_merge_tree_read_one_thread.sql

@@ -0,0 +1,16 @@
+drop table if exists t;
+
+create table t (a Int, b Int) engine = MergeTree order by (a, b) settings index_granularity = 400;
+
+insert into t select 0, 0 from numbers(50);
+insert into t select 0, 1  from numbers(350);
+insert into t select 1, 2  from numbers(400);
+insert into t select 2, 2  from numbers(400);
+insert into t select 3, 0 from numbers(100);
+
+select sleep(1) format Null; -- sleep a bit to wait possible merges after insert
+
+set max_threads = 1;
+optimize table t final;
+
+select sum(a) from t where a in (0, 3) and b = 0;