Add some comments for the feature mow (#11028)

be/src/olap/primary_key_index.h

@@ -34,6 +34,8 @@ namespace doris {
 // The primary key index is designed in a similar way like RocksDB
 // Partitioned Index, which is created in the segment file when MemTable flushes.
 // Index is stored in multiple pages to leverage the IndexedColumnWriter.
+//
+// NOTE: for now, it's only used when unique key merge-on-write property enabled.
 class PrimaryKeyIndexBuilder {
 public:
     PrimaryKeyIndexBuilder(io::FileWriter* file_writer)

be/src/olap/tablet_meta.h

@@ -240,7 +240,10 @@ class TabletMeta {
     // FIXME(cyx): Currently `cooldown_resource` is equivalent to `storage_policy`.
     io::ResourceId _cooldown_resource;
 
-    // may be true iff unique keys model.
+    // For unique key data model, the feature Merge-on-Write will leverage a primary
+    // key index and a delete-bitmap to mark duplicate keys as deleted in load stage,
+    // which can avoid the merging cost in read stage, and accelerate the aggregation
+    // query performance significantly.
     bool _enable_unique_key_merge_on_write = false;
     std::unique_ptr<DeleteBitmap> _delete_bitmap;
 
@@ -249,7 +252,8 @@ class TabletMeta {
 
 /**
  * Wraps multiple bitmaps for recording rows (row id) that are deleted or
- * overwritten.
+ * overwritten. For now, it's only used when unique key merge-on-write property
+ * enabled.
  *
  * RowsetId and SegmentId are for locating segment, Version here is a single
  * uint32_t means that at which "version" of the load causes the delete or

fe/fe-core/src/main/java/org/apache/doris/common/util/PropertyAnalyzer.java

@@ -118,6 +118,12 @@ public class PropertyAnalyzer {
     private static final double MAX_FPP = 0.05;
     private static final double MIN_FPP = 0.0001;
 
+    // For unique key data model, the feature Merge-on-Write will leverage a primary
+    // key index and a delete-bitmap to mark duplicate keys as deleted in load stage,
+    // which can avoid the merging cost in read stage, and accelerate the aggregation
+    // query performance significantly.
+    // For the detail design, see the [DISP-018](https://cwiki.apache.org/confluence/
+    // display/DORIS/DSIP-018%3A+Support+Merge-On-Write+implementation+for+UNIQUE+KEY+data+model)
     public static final String ENABLE_UNIQUE_KEY_MERGE_ON_WRITE = "enable_unique_key_merge_on_write";
 
     /**