add some comment

fe/fe-core/src/main/java/org/apache/doris/mtmv/MVCache.java

@@ -42,7 +42,6 @@ public class MVCache {
     private final Plan logicalPlan;
     // this should be shuttle expression with lineage
     private final List<NamedExpression> mvOutputExpressions;
-    // the context when parse, analyze, optimize the mv logical plan
 
     public MVCache(MTMV materializedView, Plan logicalPlan, List<NamedExpression> mvOutputExpressions) {
         this.logicalPlan = logicalPlan;

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

@@ -23,9 +23,16 @@
  * For example: after analyze plan when query or explain, we should generate materialization context.
  */
 public interface PlannerHook {
+
+    /**
+     * the hook before analyze
+     */
     default void beforeAnalyze(NereidsPlanner planner) {
     }
 
+    /**
+     * the hook after analyze
+     */
     default void afterAnalyze(NereidsPlanner planner) {
     }
 }

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

@@ -65,6 +65,8 @@ protected Plan rewriteQueryByView(MatchMode matchMode,
                 || expressionsRewritten.stream().anyMatch(expr -> !(expr instanceof NamedExpression))) {
             return null;
         }
+        // record the group id in materializationContext, and when rewrite again in
+        // the same group, bail out quickly.
         if (queryStructInfo.getOriginalPlan().getGroupExpression().isPresent()) {
             materializationContext.addMatchedGroup(
                     queryStructInfo.getOriginalPlan().getGroupExpression().get().getOwnerGroup().getGroupId());
@@ -88,10 +90,11 @@ protected boolean checkPattern(StructInfo structInfo) {
                     SUPPORTED_JOIN_TYPE_SET)) {
                 return false;
             }
-            for (Edge edge : hyperGraph.getEdges()) {
-                if (!edge.getJoin().accept(StructInfo.JOIN_PATTERN_CHECKER, SUPPORTED_JOIN_TYPE_SET)) {
-                    return false;
-                }
+        }
+        for (Edge edge : hyperGraph.getEdges()) {
+            if (!edge.getJoin().accept(StructInfo.JOIN_PATTERN_CHECKER,
+                    SUPPORTED_JOIN_TYPE_SET)) {
+                return false;
             }
         }
         return true;

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

@@ -92,6 +92,7 @@ protected List<Plan> rewrite(Plan queryPlan, CascadesContext cascadesContext) {
             }
             List<RelationMapping> queryToViewTableMappings =
                     RelationMapping.generate(queryStructInfo.getRelations(), viewStructInfo.getRelations());
+            // if any relation in query and view can not map, bail out.
             if (queryToViewTableMappings == null) {
                 return rewriteResults;
             }
@@ -161,7 +162,7 @@ protected Plan rewriteQueryByView(MatchMode matchMode,
      */
     protected List<Expression> rewriteExpression(
             List<? extends Expression> sourceExpressionsToWrite,
-            ExpressionMapping mvExpressionToMvScanExpressionMapping,
+            ExpressionMapping mvExprToMvScanExprMapping,
             SlotMapping sourceToTargetMapping) {
         // Firstly, rewrite the target plan output expression using query with inverse mapping
         // then try to use the mv expression to represent the query. if any of source expressions
@@ -175,13 +176,14 @@ protected List<Expression> rewriteExpression(
         //     transform source to:
         //        project(slot 2, 1)
         //            target
-        // generate mvSql to mvScan mvExpressionToMvScanExpressionMapping, and change mv sql expression to query based
-        ExpressionMapping expressionMappingKeySourceBased =
-                mvExpressionToMvScanExpressionMapping.keyPermute(sourceToTargetMapping.inverse());
-        List<Map<? extends Expression, ? extends Expression>> flattenExpressionMap =
-                expressionMappingKeySourceBased.flattenMap();
+        // generate mvSql to mvScan mvExprToMvScanExprMapping, and change mv sql expression to query based
+        ExpressionMapping mvExprToMvScanExprMappingKeySourceBased =
+                mvExprToMvScanExprMapping.keyPermute(sourceToTargetMapping.inverse());
+        List<Map<? extends Expression, ? extends Expression>> flattenExpressionMapping =
+                mvExprToMvScanExprMappingKeySourceBased.flattenMap();
         // view to view scan expression is 1:1 so get first element
-        Map<? extends Expression, ? extends Expression> mvSqlToMvScanMappingQueryBased = flattenExpressionMap.get(0);
+        Map<? extends Expression, ? extends Expression> mvSqlToMvScanMappingQueryBased =
+                flattenExpressionMapping.get(0);
 
         List<Expression> rewrittenExpressions = new ArrayList<>();
         for (Expression expressionToRewrite : sourceExpressionsToWrite) {

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

@@ -27,6 +27,8 @@
 
 /**
  * EquivalenceClassSetMapping
+ * This will extract the equivalence class set in EquivalenceClass and mapping set in
+ * two different EquivalenceClass.
  */
 public class EquivalenceClassSetMapping extends Mapping {
 
@@ -42,7 +44,7 @@ public static EquivalenceClassSetMapping of(Map<Set<SlotReference>, Set<SlotRefe
     }
 
     /**
-     * source equivalence set map to target equivalence set
+     * Generate source equivalence set map to target equivalence set
      */
     public static EquivalenceClassSetMapping generate(EquivalenceClass source, EquivalenceClass target) {
 

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

@@ -32,7 +32,7 @@
 import java.util.Map;
 
 /**
- * Expression and it's index mapping
+ * Expression mapping, maybe one expression map to multi expression
  */
 public class ExpressionMapping extends Mapping {
     private final Multimap<? extends Expression, ? extends Expression> expressionMapping;