diff --git a/internal/indexcoord/meta_table.go b/internal/indexcoord/meta_table.go
index 6f2b21259c..83488ead1a 100644
--- a/internal/indexcoord/meta_table.go
+++ b/internal/indexcoord/meta_table.go
@@ -163,6 +163,7 @@ func (mt *metaTable) AddIndex(indexBuildID UniqueID, req *indexpb.BuildIndexRequ
 	return mt.saveIndexMeta(meta)
 }
 
+// BuildIndex set the index state to be InProgress. It means IndexNode is building the index.
 func (mt *metaTable) BuildIndex(indexBuildID UniqueID, nodeID int64) error {
 	mt.lock.Lock()
 	defer mt.lock.Unlock()