ClickHouse/src/Storages/System/StorageSystemZeros.h

#pragma once

#include <boost/noncopyable.hpp>
#include <optional>
#include <Storages/IStorage.h>

namespace DB
{

/** Implements a table engine for the system table "zeros".
  * The table contains the only column zero UInt8.
  * From this table, you can read non-materialized zeros.
  *
  * You could also specify a limit (how many zeros to give).
  * If multithreaded is specified, zeros will be generated in several streams.
  */
class StorageSystemZeros final : public IStorage, boost::noncopyable
{
public:
    /// If even_distribution is true, numbers are distributed evenly between streams.
    /// Otherwise, streams concurrently increment atomic.
    StorageSystemZeros(const StorageID & table_id_, bool multithreaded_, std::optional<UInt64> limit_ = std::nullopt);

    std::string getName() const override { return "SystemZeros"; }

    Pipe read(
        const Names & column_names,
        const StorageSnapshotPtr & storage_snapshot,
        SelectQueryInfo & query_info,
        ContextPtr context,
        QueryProcessingStage::Enum processed_stage,
        size_t max_block_size,
        unsigned num_streams) override;

    bool hasEvenlyDistributedRead() const override { return true; }
    bool isSystemStorage() const override { return true; }
    bool supportsTransactions() const override { return true; }

private:
    bool multithreaded;
    std::optional<UInt64> limit;
};

}