Storage

• registerStorages() of registerStorages.h

• every storages must implement a read() method to return BlockInputStreams

• core classes

  databse --> table --> StorageMergeTree --> MergeTreeData --> MergeTreeDataPart


  MergeTreeBlockInputStream   ---> (MergeTreeData,MergeTreeDataPart) ---> MergeTreeReader  ----> MergeTreeReader::Stream ---> CompressedReadBufferFromFile

  MergeTreeBlockOutputStream  ---> (MergeTreeData,MergeTreeDataPart) ---> MergeTreeDataWriter ---> MergedBlockOutputStream ----> IMergedBlockOutputStream::ColumnStream ---> WriteBufferFromFile


 StorageMergeTree ---> BlockInputStreams
                   ---> BlockOutputStreamPtr
                   ---> MergeTreeDataSelectExecutor
                   ---> MergeTreeDataWriter
                   ---> MergeTreeDataMergerMutator