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#pragma once
#include "clickhouse_config.h"
#include <Storages/IStorage.h>
namespace DB
{
struct ExternalDataSourceConfiguration;
/// Storages MySQL and PostgreSQL use ConnectionPoolWithFailover and support multiple replicas.
/// This class unites multiple storages with replicas into multiple shards with replicas.
/// A query to external database is passed to one replica on each shard, the result is united.
/// Replicas on each shard have the same priority, traversed replicas are moved to the end of the queue.
/// Similar approach is used for URL storage.
class StorageExternalDistributed final : public DB::IStorage
{
public:
StorageExternalDistributed(
const StorageID & table_id_,
std::unordered_set<StoragePtr> && shards_,
const ColumnsDescription & columns_,
const ConstraintsDescription & constraints_,
const String & comment);
std::string getName() const override { return "ExternalDistributed"; }
void read(
QueryPlan & query_plan,
const Names & column_names,
const StorageSnapshotPtr & storage_snapshot,
SelectQueryInfo & query_info,
ContextPtr context,
QueryProcessingStage::Enum processed_stage,
size_t max_block_size,
size_t num_streams) override;
private:
using Shards = std::unordered_set<StoragePtr>;
Shards shards;
};
}
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