Revert "[scan](improvement) Adjust parallel scanners num (apache#38430)…

…" (apache#39046)

This reverts commit 11b332f.

be/src/pipeline/exec/scan_operator.cpp

@@ -1222,8 +1222,9 @@ Status ScanLocalState<Derived>::_start_scanners(
             state(), this, p._output_tuple_desc, p.output_row_descriptor(), scanners, p.limit(),
             state()->scan_queue_mem_limit(), _scan_dependency,
             // 1. If data distribution is ignored , we use 1 instance to scan.
-            // 2. Else, file scanner will consume much memory so we use config::doris_scanner_thread_pool_thread_num / query_parallel_instance_num scanners to scan.
-            p.ignore_data_distribution() && !p.is_file_scan_operator()
+            // 2. Else if this operator is not file scan operator, we use config::doris_scanner_thread_pool_thread_num scanners to scan.
+            // 3. Else, file scanner will consume much memory so we use config::doris_scanner_thread_pool_thread_num / query_parallel_instance_num scanners to scan.
+            p.ignore_data_distribution() || !p.is_file_scan_operator()
                     ? 1
                     : state()->query_parallel_instance_num());
     return Status::OK();