[improvement](mtmv) Optimize the materialized view hint info when explain

fe/fe-core/src/main/java/org/apache/doris/nereids/NereidsPlanner.java

@@ -535,9 +535,10 @@ public String getExplainString(ExplainOptions explainOptions) {
         String plan = "";
         String mvSummary = "";
         if (this.getPhysicalPlan() != null && cascadesContext != null) {
-            mvSummary = "\n\n========== MATERIALIZATIONS ==========\n"
-                    + MaterializationContext.toSummaryString(cascadesContext.getMaterializationContexts(),
-                    this.getPhysicalPlan());
+            mvSummary = cascadesContext.getMaterializationContexts().isEmpty() ? "" :
+                    "\n\n========== MATERIALIZATIONS ==========\n"
+                            + MaterializationContext.toSummaryString(cascadesContext.getMaterializationContexts(),
+                            this.getPhysicalPlan());
         }
         switch (explainLevel) {
             case PARSED_PLAN:
@@ -596,9 +597,10 @@ public String getExplainString(ExplainOptions explainOptions) {
             default:
                 plan = super.getExplainString(explainOptions);
                 plan += mvSummary;
+                plan += "\n\n\n========== STATISTICS ==========\n";
                 if (statementContext != null) {
                     if (statementContext.isHasUnknownColStats()) {
-                        plan += "\n\nStatistics\n planed with unknown column statistics\n";
+                        plan += "planed with unknown column statistics\n";
                     }
                 }
         }

fe/fe-core/src/main/java/org/apache/doris/nereids/rules/exploration/mv/MaterializationContext.java

@@ -408,7 +408,7 @@ public Void visitPhysicalRelation(PhysicalRelation physicalRelation, Void contex
     }
 
     private static String generateIdentifierName(List<String> qualifiers) {
-        return String.join("#", qualifiers);
+        return String.join(".", qualifiers);
     }
 
     @Override

regression-test/suites/nereids_rules_p0/mv/same_name/sync_async_same_name.groovy

@@ -165,7 +165,7 @@ suite("sync_async_same_name") {
         check {result ->
             def splitResult = result.split("MaterializedViewRewriteFail")
             splitResult.length == 2 ? splitResult[0].contains(common_mv_name)
-                    && splitResult[0].contains("orders#${common_mv_name}") : false
+                    && splitResult[0].contains("orders.${common_mv_name}") : false
         }
     }