Attempt to improve performance of replication queue processing [#METR-22352].

dbms/include/DB/Storages/StorageReplicatedMergeTree.h

@@ -419,6 +419,13 @@ private:
 	  */
 	String findReplicaHavingPart(const String & part_name, bool active);
 
+	/** Find replica having specified part or any part that covers it.
+	  * If active = true, consider only active replicas.
+	  * If found, returns replica name and set 'out_covering_part_name' to name of found largest covering part.
+	  * If not found, returns empty string.
+	  */
+	String findReplicaHavingCoveringPart(const String & part_name, bool active, String & out_covering_part_name);
+
 	/** Скачать указанный кусок с указанной реплики.
 	  * Если to_detached, то кусок помещается в директорию detached.
 	  * Если quorum != 0, то обновляется узел для отслеживания кворума.

dbms/src/Storages/StorageReplicatedMergeTree.cpp

@@ -1208,7 +1208,8 @@ bool StorageReplicatedMergeTree::executeLogEntry(const LogEntry & entry, Backgro
 
 		try
 		{
-			replica = findReplicaHavingPart(entry.new_part_name, true);
+			String covering_part;
+			replica = findReplicaHavingCoveringPart(entry.new_part_name, true, covering_part);
 
 			if (replica.empty() && entry.type == LogEntry::ATTACH_PART)
 			{
@@ -1223,7 +1224,7 @@ bool StorageReplicatedMergeTree::executeLogEntry(const LogEntry & entry, Backgro
 
 				/// Подождём, пока реплика-инициатор подцепит кусок.
 				waitForReplicaToProcessLogEntry(entry.source_replica, entry);
-				replica = findReplicaHavingPart(entry.new_part_name, true);
+				replica = findReplicaHavingCoveringPart(entry.new_part_name, true, covering_part);
 			}
 
 			if (replica.empty())
@@ -1351,11 +1352,11 @@ bool StorageReplicatedMergeTree::executeLogEntry(const LogEntry & entry, Backgro
 				if (replica.empty())
 				{
 					ProfileEvents::increment(ProfileEvents::ReplicatedPartFailedFetches);
-					throw Exception("No active replica has part " + entry.new_part_name, ErrorCodes::NO_REPLICA_HAS_PART);
+					throw Exception("No active replica has part " + entry.new_part_name + " or covering part", ErrorCodes::NO_REPLICA_HAS_PART);
 				}
 			}
 
-			fetchPart(entry.new_part_name, zookeeper_path + "/replicas/" + replica, false, entry.quorum);
+			fetchPart(covering_part, zookeeper_path + "/replicas/" + replica, false, entry.quorum);
 
 			if (entry.type == LogEntry::MERGE_PARTS)
 				ProfileEvents::increment(ProfileEvents::ReplicatedPartFetchesOfMerged);
@@ -1872,7 +1873,7 @@ String StorageReplicatedMergeTree::findReplicaHavingPart(const String & part_nam
 	auto zookeeper = getZooKeeper();
 	Strings replicas = zookeeper->getChildren(zookeeper_path + "/replicas");
 
-	/// Из реплик, у которых есть кусок, выберем одну равновероятно.
+	/// Select replicas in uniformly random order.
 	std::random_shuffle(replicas.begin(), replicas.end());
 
 	for (const String & replica : replicas)
@@ -1884,7 +1885,45 @@ String StorageReplicatedMergeTree::findReplicaHavingPart(const String & part_nam
 		/// Конечно, реплика может перестать быть активной или даже перестать существовать после возврата из этой функции.
 	}
 
-	return "";
+	return {};
+}
+
+
+String StorageReplicatedMergeTree::findReplicaHavingCoveringPart(const String & part_name, bool active, String & out_covering_part_name)
+{
+	auto zookeeper = getZooKeeper();
+	Strings replicas = zookeeper->getChildren(zookeeper_path + "/replicas");
+
+	/// Select replicas in uniformly random order.
+	std::random_shuffle(replicas.begin(), replicas.end());
+
+	for (const String & replica : replicas)
+	{
+		if (active && !zookeeper->exists(zookeeper_path + "/replicas/" + replica + "/is_active"))
+			continue;
+
+		String largest_part_found;
+		Strings parts = zookeeper->getChildren(zookeeper_path + "/replicas/" + replica + "/parts");
+		for (const String & part_on_replica : parts)
+		{
+			if (part_on_replica == part_name || ActiveDataPartSet::contains(part_on_replica, part_name))
+			{
+				if (largest_part_found.empty()
+					|| ActiveDataPartSet::contains(part_on_replica, largest_part_found))
+				{
+					largest_part_found = part_on_replica;
+				}
+			}
+		}
+
+		if (!largest_part_found.empty())
+		{
+			out_covering_part_name = largest_part_found;
+			return replica_name;
+		}
+	}
+
+	return {};
 }