text: #pragma once
  
  #include <atomic>
  #include <optional>
  #include <mutex>
  
  #include <ext/shared_ptr_helper.h>
  
  #include <Core/NamesAndTypes.h>
  #include <Storages/IStorage.h>
  #include <DataStreams/IBlockOutputStream.h>
  
  #include <Common/MultiVersion.h>
  
  namespace DB
  {
  
  /** Implements storage in the RAM.
    * Suitable for temporary data.
    * It does not support keys.
    * Data is stored as a set of blocks and is not stored anywhere else.
    */
  class StorageMemory final : public ext::shared_ptr_helper<StorageMemory>, public IStorage
  {
  friend class MemoryBlockOutputStream;
  friend struct ext::shared_ptr_helper<StorageMemory>;
  
  public:
      String getName() const override { return "Memory"; }
  
      size_t getSize() const { return data.get()->size(); }
  
      Pipe read(
          const Names & column_names,
          const StorageMetadataPtr & /*metadata_snapshot*/,
          SelectQueryInfo & query_info,
          const Context & context,
          QueryProcessingStage::Enum processed_stage,
          size_t max_block_size,
          unsigned num_streams) override;
  
      bool supportsParallelInsert() const override { return true; }
      bool supportsSubcolumns() const override { return true; }
  
      /// Smaller blocks (e.g. 64K rows) are better for CPU cache.
      bool prefersLargeBlocks() const override { return false; }
  
      bool hasEvenlyDistributedRead() const override { return true; }
  
      BlockOutputStreamPtr write(const ASTPtr & query, const StorageMetadataPtr & metadata_snapshot, const Context & context) override;
  
      void drop() override;
  
      void checkMutationIsPossible(const MutationCommands & commands, const Settings & settings) const override;
      void mutate(const MutationCommands & commands, const Context & context) override;
  
      void truncate(const ASTPtr &, const StorageMetadataPtr &, const Context &, TableExclusiveLockHolder &) override;
  
      std::optional<UInt64> totalRows(const Settings &) const override;
      std::optional<UInt64> totalBytes(const Settings &) const override;
  
      /** Delays initialization of StorageMemory::read() until the first read is actually happen.
        * Usually, fore code like this:
        *
        *     auto out = StorageMemory::write();
        *     auto in = StorageMemory::read();
        *     out->write(new_data);
        *
        * `new_data` won't appear into `in`.
        *  However, if delayReadForGlobalSubqueries is called, first read from `in` will check for new_data and return it.
        *
        *
        * Why is delayReadForGlobalSubqueries needed?
        *
        * The fact is that when processing a query of the form
        *  SELECT ... FROM remote_test WHERE column GLOBAL IN (subquery),
        *  if the distributed remote_test table contains localhost as one of the servers,
        *  the query will be interpreted locally again (and not sent over TCP, as in the case of a remote server).
        *
        * The query execution pipeline will be:
        * CreatingSets
        *  subquery execution, filling the temporary table with _data1 (1)
        *  CreatingSets
        *   reading from the table _data1, creating the set (2)
        *   read from the table subordinate to remote_test.
        *
        * (The second part of the pipeline under CreateSets is a reinterpretation of the query inside StorageDistributed,
        *  the query differs in that the database name and tables are replaced with subordinates, and the subquery is replaced with _data1.)
        *
        * But when creating the pipeline, when creating the source (2), it will be found that the _data1 table is empty
        *  (because the query has not started yet), and empty source will be returned as the source.
        * And then, when the query is executed, an empty set will be created in step (2).
        *
        * Therefore, we make the initialization of step (2) delayed
        *  - so that it does not occur until step (1) is completed, on which the table will be populated.
        */
      void delayReadForGlobalSubqueries() { delay_read_for_global_subqueries = true; }
  
  private:
      /// MultiVersion data storage, so that we can copy the list of blocks to readers.
  
      MultiVersion<Blocks> data;
  
      mutable std::mutex mutex;
  
      bool delay_read_for_global_subqueries = false;
  
      std::atomic<size_t> total_size_bytes = 0;
      std::atomic<size_t> total_size_rows = 0;
  
      bool compress;
  
  protected:
      StorageMemory(
          const StorageID & table_id_,
          ColumnsDescription columns_description_,
          ConstraintsDescription constraints_,
          bool compress_ = false);
  };
  
  }
  