From 7d630b6b1bee7839d295eb5a548e17e94012fb93 Mon Sep 17 00:00:00 2001
From: Igor Nikonov <igor@clickhouse.com>
Date: Sun, 10 Sep 2023 20:38:17 +0000
Subject: [PATCH] Added comments

---
 src/Interpreters/ClusterProxy/SelectStreamFactory.cpp | 3 +++
 src/Interpreters/ClusterProxy/executeQuery.cpp        | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/src/Interpreters/ClusterProxy/SelectStreamFactory.cpp b/src/Interpreters/ClusterProxy/SelectStreamFactory.cpp
index 2f598fda3a9..3935028f27c 100644
--- a/src/Interpreters/ClusterProxy/SelectStreamFactory.cpp
+++ b/src/Interpreters/ClusterProxy/SelectStreamFactory.cpp
@@ -147,6 +147,9 @@ void SelectStreamFactory::createForShard(
         return;
     });
 
+    // prefer_localhost_replica is not effective in case of parallel replicas
+    // (1) prefer_localhost_replica is about choosing one replica on a shard
+    // (2) parallel replica coordinator has own logic to choose replicas to read from
     if (settings.prefer_localhost_replica && shard_info.isLocal() && !parallel_replicas_enabled)
     {
         StoragePtr main_table_storage;
diff --git a/src/Interpreters/ClusterProxy/executeQuery.cpp b/src/Interpreters/ClusterProxy/executeQuery.cpp
index 401eea7c39a..bd8f5cea7f3 100644
--- a/src/Interpreters/ClusterProxy/executeQuery.cpp
+++ b/src/Interpreters/ClusterProxy/executeQuery.cpp
@@ -211,6 +211,8 @@ void executeQuery(
             }
         }
 
+        // decide for each shard if parallel reading from replicas should be enabled
+        // according to settings and number of replicas declared per shard
         bool parallel_replicas_enabled = false;
         if (shard_info.shard_num > 0 && shard_info.shard_num <= cluster->getShardsAddresses().size())
         {