[feature](load) enable memtable on sink node by default (#30372)

docs/en/docs/advanced/variables.md

@@ -693,7 +693,7 @@ Note that the comment must start with /*+ and can only follow the SELECT.
 * `enable_memtable_on_sink_node`
 
   <version since="2.1.0">
-  Whether to enable MemTable on DataSink node when loading data, default is false.
+  Whether to enable MemTable on DataSink node when loading data, default is true.
   </version>
 
   Build MemTable on DataSink node, and send segments to other backends through brpc streaming.

docs/zh-CN/docs/advanced/variables.md

@@ -681,7 +681,7 @@ try (Connection conn = DriverManager.getConnection("jdbc:mysql://127.0.0.1:9030/
 * `enable_memtable_on_sink_node`
 
   <version since="2.1.0">
-  是否在数据导入中启用 MemTable 前移，默认为 false
+  是否在数据导入中启用 MemTable 前移，默认为 true
   </version>
 
   在 DataSink 节点上构建 MemTable，并通过 brpc streaming 发送 segment 到其他 BE。

fe/fe-core/src/main/java/org/apache/doris/qe/SessionVariable.java

@@ -1393,7 +1393,7 @@ public class SessionVariable implements Serializable, Writable {
     public boolean truncateCharOrVarcharColumns = false;
 
     @VariableMgr.VarAttr(name = ENABLE_MEMTABLE_ON_SINK_NODE, needForward = true)
-    public boolean enableMemtableOnSinkNode = false;
+    public boolean enableMemtableOnSinkNode = true;
 
     @VariableMgr.VarAttr(name = LOAD_STREAM_PER_NODE)
     public int loadStreamPerNode = 20;