Merge pull request #58526 from ClickHouse/backport/23.11/58505

Backport #58505 to 23.11: MergeTreePrefetchedReadPool disable for LIMIT only queries
ClickHouse · Jan 5, 2024 · e79d840 · e79d840
2 parents 436840c + f5dc152
commit e79d840
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Showing 3 changed files with 19 additions and 3 deletions.
diff --git a/src/Interpreters/InterpreterSelectQuery.cpp b/src/Interpreters/InterpreterSelectQuery.cpp
@@ -2470,7 +2470,12 @@ void InterpreterSelectQuery::executeFetchColumns(QueryProcessingStage::Enum proc
             max_block_size = std::max<UInt64>(1, max_block_limited);
             max_threads_execute_query = max_streams = 1;
         }
-        if (max_block_limited < local_limits.local_limits.size_limits.max_rows)
+        if (local_limits.local_limits.size_limits.max_rows != 0)
+        {
+            if (max_block_limited < local_limits.local_limits.size_limits.max_rows)
+                query_info.limit = max_block_limited;
+        }
+        else
         {
             query_info.limit = max_block_limited;
         }

diff --git a/src/Planner/PlannerJoinTree.cpp b/src/Planner/PlannerJoinTree.cpp
@@ -645,7 +645,12 @@ JoinTreeQueryPlan buildQueryPlanForTableExpression(QueryTreeNodePtr table_expres
                     max_threads_execute_query = 1;
                 }
 
-                if (max_block_size_limited < select_query_info.local_storage_limits.local_limits.size_limits.max_rows)
+                if (select_query_info.local_storage_limits.local_limits.size_limits.max_rows != 0)
+                {
+                    if (max_block_size_limited < select_query_info.local_storage_limits.local_limits.size_limits.max_rows)
+                        table_expression_query_info.limit = max_block_size_limited;
+                }
+                else
                 {
                     table_expression_query_info.limit = max_block_size_limited;
                 }

diff --git a/src/Processors/QueryPlan/ReadFromMergeTree.cpp b/src/Processors/QueryPlan/ReadFromMergeTree.cpp
@@ -418,7 +418,13 @@ Pipe ReadFromMergeTree::readFromPool(
         && settings.allow_prefetched_read_pool_for_local_filesystem
         && MergeTreePrefetchedReadPool::checkReadMethodAllowed(reader_settings.read_settings.local_fs_method);
 
-    if (allow_prefetched_remote || allow_prefetched_local)
+    /** Do not use prefetched read pool if query is trivial limit query.
+      * Because time spend during filling per thread tasks can be greater than whole query
+      * execution for big tables with small limit.
+      */
+    bool use_prefetched_read_pool = query_info.limit == 0 && (allow_prefetched_remote || allow_prefetched_local);
+
+    if (use_prefetched_read_pool)
     {
         pool = std::make_shared<MergeTreePrefetchedReadPool>(
             std::move(parts_with_range),