Adjusting num_streams by expected work in StorageS3

src/Storages/StorageS3.cpp

@@ -180,6 +180,13 @@ public:
         return nextAssumeLocked();
     }
 
+    size_t objectsCount()
+    {
+        assert(outcome_future.valid());
+        first_outcome = outcome_future.get();
+        return first_outcome->GetResult().GetContents().size();
+    }
+
     ~Impl()
     {
         list_objects_pool.wait();
@@ -225,8 +232,17 @@ private:
     {
         buffer.clear();
 
-        assert(outcome_future.valid());
+        ListObjectsOutcome outcome;
-        auto outcome = outcome_future.get();
+        if (unlikely(first_outcome))
+        {
+            outcome = std::move(*first_outcome);
+            first_outcome = std::nullopt;
+        }
+        else
+        {
+            assert(outcome_future.valid());
+            outcome = outcome_future.get();
+        }
 
         if (!outcome.IsSuccess())
         {
@@ -343,6 +359,7 @@ private:
     ThreadPool list_objects_pool;
     ThreadPoolCallbackRunner<ListObjectsOutcome> list_objects_scheduler;
     std::future<ListObjectsOutcome> outcome_future;
+    std::optional<ListObjectsOutcome> first_outcome;  /// the result will be set by `estimatedKeysCount`
     std::function<void(FileProgress)> file_progress_callback;
 };
 
@@ -364,6 +381,11 @@ StorageS3Source::KeyWithInfo StorageS3Source::DisclosedGlobIterator::next()
     return pimpl->next();
 }
 
+size_t StorageS3Source::DisclosedGlobIterator::estimatedKeysCount()
+{
+    return pimpl->objectsCount();
+}
+
 class StorageS3Source::KeysIterator::Impl : WithContext
 {
 public:
@@ -425,6 +447,11 @@ public:
         return {key, info};
     }
 
+    size_t objectsCount()
+    {
+        return keys.size();
+    }
+
 private:
     Strings keys;
     std::atomic_size_t index = 0;
@@ -459,6 +486,43 @@ StorageS3Source::KeyWithInfo StorageS3Source::KeysIterator::next()
     return pimpl->next();
 }
 
+size_t StorageS3Source::KeysIterator::estimatedKeysCount()
+{
+    return pimpl->objectsCount();
+}
+
+StorageS3Source::ReadTaskIterator::ReadTaskIterator(
+    const DB::ReadTaskCallback & callback_,
+    const size_t max_threads_count)
+    : callback(callback_)
+{
+    ThreadPool pool(CurrentMetrics::StorageS3Threads, CurrentMetrics::StorageS3ThreadsActive, max_threads_count);
+    auto pool_scheduler = threadPoolCallbackRunner<String>(pool, "ReadTaskIteratorPrefetch");
+
+    std::vector<std::future<String>> keys;
+    for (size_t i = 0; i < max_threads_count; ++i)
+        keys.push_back(pool_scheduler([this] { return callback(); }, Priority{}));
+
+    pool.wait();
+    buffer.reserve(max_threads_count);
+    for (auto & key_future : keys)
+        buffer.emplace_back(key_future.get(), std::nullopt);
+}
+
+StorageS3Source::KeyWithInfo StorageS3Source::ReadTaskIterator::next()
+{
+    size_t current_index = index.fetch_add(1, std::memory_order_relaxed);
+    if (current_index >= buffer.size())
+        return {callback(), {}};
+
+    return buffer[current_index];
+}
+
+size_t StorageS3Source::ReadTaskIterator::estimatedKeysCount()
+{
+    return buffer.size();
+}
+
 StorageS3Source::StorageS3Source(
     const ReadFromFormatInfo & info,
     const String & format_,
@@ -965,7 +1029,7 @@ std::shared_ptr<StorageS3Source::IIterator> StorageS3::createFileIterator(
 {
     if (distributed_processing)
     {
-        return std::make_shared<StorageS3Source::ReadTaskIterator>(local_context->getReadTaskCallback());
+        return std::make_shared<StorageS3Source::ReadTaskIterator>(local_context->getReadTaskCallback(), local_context->getSettingsRef().max_threads);
     }
     else if (configuration.withGlobs())
     {
@@ -1017,6 +1081,9 @@ Pipe StorageS3::read(
     std::shared_ptr<StorageS3Source::IIterator> iterator_wrapper = createFileIterator(
         query_configuration, distributed_processing, local_context, query_info.query, virtual_columns, nullptr, local_context->getFileProgressCallback());
 
+    size_t estimated_keys_count = iterator_wrapper->estimatedKeysCount();
+    num_streams = std::min(num_streams, estimated_keys_count);
+
     auto read_from_format_info = prepareReadingFromFormat(column_names, storage_snapshot, supportsSubsetOfColumns(local_context), getVirtuals());
     bool need_only_count = (query_info.optimize_trivial_count || read_from_format_info.requested_columns.empty())
         && local_context->getSettingsRef().optimize_count_from_files;
@@ -1024,6 +1091,7 @@ Pipe StorageS3::read(
     const size_t max_threads = local_context->getSettingsRef().max_threads;
     const size_t max_parsing_threads = num_streams >= max_threads ? 1 : (max_threads / num_streams);
 
+    pipes.reserve(num_streams);
     for (size_t i = 0; i < num_streams; ++i)
     {
         pipes.emplace_back(std::make_shared<StorageS3Source>(

src/Storages/StorageS3.h

@@ -60,6 +60,10 @@ public:
         virtual ~IIterator() = default;
         virtual KeyWithInfo next() = 0;
 
+        /// Estimates how many streams we need to process all files.
+        /// If keys count >= max_threads_count, the returned number may not represent the actual number of the keys.
+        virtual size_t estimatedKeysCount() = 0;
+
         KeyWithInfo operator ()() { return next(); }
     };
 
@@ -77,6 +81,7 @@ public:
             std::function<void(FileProgress)> progress_callback_ = {});
 
         KeyWithInfo next() override;
+        size_t estimatedKeysCount() override;
 
     private:
         class Impl;
@@ -100,6 +105,7 @@ public:
             std::function<void(FileProgress)> progress_callback_ = {});
 
         KeyWithInfo next() override;
+        size_t estimatedKeysCount() override;
 
     private:
         class Impl;
@@ -110,11 +116,15 @@ public:
     class ReadTaskIterator : public IIterator
     {
     public:
-        explicit ReadTaskIterator(const ReadTaskCallback & callback_) : callback(callback_) {}
+        explicit ReadTaskIterator(const ReadTaskCallback & callback_, const size_t max_threads_count);
 
-        KeyWithInfo next() override { return {callback(), {}}; }
+        KeyWithInfo next() override;
+        size_t estimatedKeysCount() override;
 
     private:
+        KeysWithInfo buffer;
+        std::atomic_size_t index = 0;
+
         ReadTaskCallback callback;
     };