weizuo93 opened a new issue #4797:
URL: https://github.com/apache/incubator-doris/issues/4797
MemTable flush can be activated by three situations：
A. The size of MemTable reach to the flush threshold
`config::write_buffer_size`;
B. Data load is finished and DeltaWriter needs to be closed；
C. Memory consumption exceeds the limit and MemTable needs to flush to
reduce memory usage.
A large number of small segment files would be generated due to "situation
B" above when small batch data (far less than `config::write_buffer_size`) are
loaded frequently , which will lead to lower efficiency for scan operations.
We can optimize MemTable flush mechanism like this:
(1) Maintain a `vector<MemTable>` for each tablet;
(2) When close DeltaWriter and flush MemTable for a tablet, do not reset the
Memtable if there is no flush operation before for this tablet in the data load
and push the MemTable into `vector<MemTable>`;
(3) When next flush operation for this tablet comming, judge whether the
flush is activated by :
a. "situation A": Merge all MemTable in `vector<MemTable>` into
current MemTable , flush the merged MemTable , delete all the rowset
corresponding to MemTable in `vector<MemTable>` and clear `vector<MemTable>`;
b. "situation B": If the total size of MemTable in
`vector<MemTable>`and current MemTable reach threshold
`config::write_buffer_size`, merge all MemTable in `vector<MemTable>` into
current MemTable, flush the merged MemTable, delete all the rowset
corresponding to MemTable in `vector<MemTable>` and clear `vector<MemTable>`;
If the total size of MemTable in `vector<MemTable>`and current MemTable is
less than the threshold `config::write_buffer_size`, push the MemTable into
`vector<MemTable>` and only flush the current MemTable;
c. "situation C": flush the current MemTable 、reset all the MemTable
in `vector<MemTable>` and clear `vector<MemTable>`.
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