Yun Tang created FLINK-13034:
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Summary: Improve the performance when checking whether mapstate is
empty for RocksDBStateBackend
Key: FLINK-13034
URL: https://issues.apache.org/jira/browse/FLINK-13034
Project: Flink
Issue Type: Improvement
Components: Runtime / State Backends
Reporter: Yun Tang
Assignee: Yun Tang
Currently, there existed several scenarios to check whether map state is empty
in Flink source code,
e.g.[TemporalRowTimeJoinOperator|https://github.com/apache/flink/blob/8315f38e89f897e32cfa0f23990cb3fb44db0d72/flink-table/flink-table-runtime-blink/src/main/java/org/apache/flink/table/runtime/join/temporal/TemporalRowTimeJoinOperator.java#L192],
[AbstractRowTimeUnboundedPrecedingOver|#L160)].
Developers would use below command to check whether the map state is empty:
{code:java}
boolean noRecordsToProcess = !inputState.keys().iterator().hasNext();
{code}
However, if we use {{RocksDBStateBackend}},
{{inputState.keys().iterator().hasNext()}} would actually call 1 {{seek}} and
128 {{next}} actions in
[RocksDBMapState|https://github.com/apache/flink/blob/8315f38e89f897e32cfa0f23990cb3fb44db0d72/flink-state-backends/flink-statebackend-rocksdb/src/main/java/org/apache/flink/contrib/streaming/state/RocksDBMapState.java#L483],
in which the redundant {{next}} actions are not what we want.
I have two options to improve this:
* Modify {{RocksDBMapState}} back to previous design which would first load
one element and then load more elements in the follow-up queries. However, this
would effect the performance of other map state methods.
* Add a {{isEmpty()}} method in the public evolving interface {{MapState}}, so
that we could use it to check whether the map state is empty without any
redundant RocksDB actions.
I prefer to the 2nd option.
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