[Enhancement]Set the parallelism of low-cardinality field tasks to 1 (#…

…31249) (#31314)

Signed-off-by: trueeyu <[email protected]>

fe/fe-core/src/main/java/com/starrocks/statistic/StatisticExecutor.java

@@ -174,6 +174,9 @@ public static Pair<List<TStatisticData>, Status> queryDictSync(Long dbId, Long t
                 " from " + StatisticUtils.quoting(catalogName, db.getOriginName(), table.getName()) + " [_META_]";
 
         ConnectContext context = StatisticUtils.buildConnectContext();
+        // The parallelism degree of low-cardinality dict collect task is uniformly set to 1 to
+        // prevent collection tasks from occupying a large number of be execution threads and scan threads.
+        context.getSessionVariable().setPipelineDop(1);
         context.setThreadLocalInfo();
         StatementBase parsedStmt = SqlParser.parseOneWithStarRocksDialect(sql, context.getSessionVariable());