Prevent overwriting existing payload with empty Payload (#2716)

* do not overwrite existing payload with empty payload

* lint

* add comments

hotshot-query-service/src/data_source/storage/sql/transaction.rs

@@ -496,8 +496,13 @@ where
         // Similarly, we can initialize the payload table with a null payload, which can help us
         // distinguish between blocks that haven't been produced yet and blocks we haven't received
         // yet when answering queries.
-        self.upsert("payload", ["height"], ["height"], [(height as i64,)])
-            .await?;
+        // We don't overwrite the payload if it already exists.
+        // During epoch transition in PoS, the same height block is sent multiple times.
+        // The first block may have the payload, but subsequent blocks might be missing it.
+        // Overwriting would cause the payload to be lost since the block height is the same
+        let query = query("INSERT INTO payload (height) VALUES ($1) ON CONFLICT DO NOTHING")
+            .bind(height as i64);
+        query.execute(self.as_mut()).await?;
 
         // Finally, we insert the leaf itself, which references the header row we created.
         // Serialize the full leaf and QC to JSON for easy storage.