change default value

src/Core/Settings.h

@@ -409,7 +409,7 @@ class IColumn;
     M(UInt64, max_rows_to_sort, 0, "If more than the specified amount of records have to be processed for ORDER BY operation, the behavior will be determined by the 'sort_overflow_mode' which by default is - throw an exception", 0) \
     M(UInt64, max_bytes_to_sort, 0, "If more than the specified amount of (uncompressed) bytes have to be processed for ORDER BY operation, the behavior will be determined by the 'sort_overflow_mode' which by default is - throw an exception", 0) \
     M(OverflowMode, sort_overflow_mode, OverflowMode::THROW, "What to do when the limit is exceeded.", 0) \
-    M(UInt64, prefer_external_sort_block_bytes, 0, "Prefer maximum block bytes for external sort, reduce the memory usage during merging.", 0) \
+    M(UInt64, prefer_external_sort_block_bytes, DEFAULT_BLOCK_SIZE * 256, "Prefer maximum block bytes for external sort, reduce the memory usage during merging.", 0) \
     M(UInt64, max_bytes_before_external_sort, 0, "If memory usage during ORDER BY operation is exceeding this threshold in bytes, activate the 'external sorting' mode (spill data to disk). Recommended value is half of available system memory.", 0) \
     M(UInt64, max_bytes_before_remerge_sort, 1000000000, "In case of ORDER BY with LIMIT, when memory usage is higher than specified threshold, perform additional steps of merging blocks before final merge to keep just top LIMIT rows.", 0) \
     M(Float, remerge_sort_lowered_memory_bytes_ratio, 2., "If memory usage after remerge does not reduced by this ratio, remerge will be disabled.", 0) \

src/Core/SettingsChangesHistory.h

@@ -85,7 +85,6 @@ namespace SettingsChangesHistory
 /// It's used to implement `compatibility` setting (see https://github.com/ClickHouse/ClickHouse/issues/35972)
 static std::map<ClickHouseVersion, SettingsChangesHistory::SettingsChanges> settings_changes_history =
 {
-    {"24.5", {{"prefer_external_sort_block_bytes", 0, 0, "Prefer maximum block bytes for external sort, reduce the memory usage during merging."}},
     {"24.4", {{"input_format_json_throw_on_bad_escape_sequence", true, true, "Allow to save JSON strings with bad escape sequences"},
               {"ignore_drop_queries_probability", 0, 0, "Allow to ignore drop queries in server with specified probability for testing purposes"},
               {"lightweight_deletes_sync", 2, 2, "The same as 'mutation_sync', but controls only execution of lightweight deletes"},
@@ -94,6 +93,7 @@ static std::map<ClickHouseVersion, SettingsChangesHistory::SettingsChanges> sett
               {"input_format_hive_text_allow_variable_number_of_columns", false, true, "Ignore extra columns in Hive Text input (if file has more columns than expected) and treat missing fields in Hive Text input as default values."},
               {"first_day_of_week", "Monday", "Monday", "Added a setting for the first day of the week for date/time functions"},
               {"temporary_data_in_cache_reserve_space_wait_lock_timeout_milliseconds", (10 * 60 * 1000), (10 * 60 * 1000), "Wait time to lock cache for sapce reservation in temporary data in filesystem cache"},
+              {"prefer_external_sort_block_bytes", 0, DEFAULT_BLOCK_SIZE * 256, "Prefer maximum block bytes for external sort, reduce the memory usage during merging."}
     }},
     {"24.3", {{"s3_connect_timeout_ms", 1000, 1000, "Introduce new dedicated setting for s3 connection timeout"},
               {"allow_experimental_shared_merge_tree", false, true, "The setting is obsolete"},

src/Processors/Transforms/MergeSortingTransform.cpp

@@ -175,10 +175,11 @@ void MergeSortingTransform::consume(Chunk chunk)
         if (max_merged_block_size > 0)
         {
             auto avg_row_bytes = sum_bytes_in_blocks / sum_rows_in_blocks;
-            /// The memory usage of the block does not exceed max block bytes, and the number of rows in the block is not less than 128.
-            /// If the max_merged_block_size is less than 128, then the new block size is not less than max_merged_block_size
-            auto min_block_size = std::min(128UL, max_merged_block_size);
-            max_merged_block_size = std::max(std::min(max_merged_block_size, max_block_bytes / avg_row_bytes), min_block_size);
+            /// max_merged_block_size >= 128
+            max_merged_block_size = std::max(std::min(max_merged_block_size, max_block_bytes / avg_row_bytes), 128UL);
+            /// when max_block_size < 128, use max_block_size.
+            /// max_block_size still works.
+            max_merged_block_size = std::min(this->max_merged_block_size, max_merged_block_size);
         }
         merge_sorter = std::make_unique<MergeSorter>(header_without_constants, std::move(chunks), description, max_merged_block_size, limit);
         auto current_processor = std::make_shared<BufferingToFileTransform>(header_without_constants, tmp_stream, log);