Make system.replicas parallel

src/Core/Settings.h

@@ -155,6 +155,8 @@ static constexpr UInt64 operator""_GiB(unsigned long long value)
     \
     M(Bool, allow_experimental_parallel_reading_from_replicas, false, "If true, ClickHouse will send a SELECT query to all replicas of a table. It will work for any kind on MergeTree table.", 0) \
     \
+    M(UInt64, system_replicas_fetch_threads, 16, "The maximum number of threads to fetch data for system.replicas table.", 0) \
+    \
     M(Bool, skip_unavailable_shards, false, "If true, ClickHouse silently skips unavailable shards and nodes unresolvable through DNS. Shard is marked as unavailable when none of the replicas can be reached.", 0) \
     \
     M(UInt64, parallel_distributed_insert_select, 0, "Process distributed INSERT SELECT query in the same cluster on local tables on every shard; if set to 1 - SELECT is executed on each shard; if set to 2 - SELECT and INSERT are executed on each shard", 0) \

src/Storages/System/StorageSystemReplicas.cpp

@@ -151,14 +151,32 @@ Pipe StorageSystemReplicas::read(
 
     MutableColumns res_columns = storage_snapshot->metadata->getSampleBlock().cloneEmptyColumns();
 
-    for (size_t i = 0, size = col_database->size(); i < size; ++i)
+    auto settings = context->getSettingsRef();
+    size_t thread_pool_size = settings.system_replicas_fetch_threads;
+
+    if (settings.max_threads != 0)
+        thread_pool_size = std::min(thread_pool_size, static_cast<size_t>(settings.max_threads));
+
+    ThreadPool thread_pool(thread_pool_size);
+
+    size_t tables_size = col_database->size();
+    std::vector<StorageReplicatedMergeTree::Status> statuses(tables_size);
+
+    for (size_t i = 0; i < tables_size; ++i)
+    {
+        thread_pool.scheduleOrThrowOnError([i, &statuses, &replicated_tables, &col_database, &col_table, &with_zk_fields]
         {
-        StorageReplicatedMergeTree::Status status;
             dynamic_cast<StorageReplicatedMergeTree &>(
                 *replicated_tables
                     [(*col_database)[i].safeGet<const String &>()]
-                [(*col_table)[i].safeGet<const String &>()]).getStatus(status, with_zk_fields);
+                    [(*col_table)[i].safeGet<const String &>()]).getStatus(statuses[i], with_zk_fields);
+        });
+    }
 
+    thread_pool.wait();
+
+    for (const auto & status: statuses)
+    {
         size_t col_num = 3;
         res_columns[col_num++]->insert(status.is_leader);
         res_columns[col_num++]->insert(status.can_become_leader);