ClickHouse/dbms/include/DB/Storages/MergeTree/MergeSelector.h
2016-10-30 06:39:28 +03:00

60 lines
1.7 KiB
C++

#pragma once
#include <cstddef>
#include <vector>
#include <functional>
namespace DB
{
/** Interface of algorithm to select data parts to merge
* (merge is also known as "compaction").
* Following properties depend on it:
*
* 1. Number of data parts at some moment in time.
* If parts are merged frequently, then data will be represented by lower number of parts, in average,
* but with cost of higher write amplification.
*
* 2. Write amplification ratio: how much times, on average, source data was written
* (during initial writes and followed merges).
*
* Number of parallel merges are controlled outside of scope of this interface.
*/
class IMergeSelector
{
public:
/// Information about data part relevant to merge selecting strategy.
struct Part
{
/// Size of data part in bytes.
size_t size;
/// How old this data part in seconds.
time_t age;
/// Depth of tree of merges by which this part was created. New parts has zero level.
unsigned level;
/// Opaque pointer to avoid dependencies (it is not possible to do forward declaration of typedef).
const void * data;
};
/// Parts are belong to partitions. Only parts within same partition could be merged.
using PartsInPartition = std::vector<Part>;
/// Parts are in some specific order. Parts could be merged only in contiguous ranges.
using Partitions = std::vector<PartsInPartition>;
/** Function could be called at any frequency and it must decide, should you do any merge at all.
* If better not to do any merge, it returns empty result.
*/
virtual PartsInPartition select(
const Partitions & partitions,
const size_t max_total_size_to_merge) = 0;
virtual ~IMergeSelector() {}
};
}