JingsongLi opened a new pull request #9876: [FLINK-14134][table] Introduce 
LimitableTableSource for optimizing limit
URL: https://github.com/apache/flink/pull/9876
 
 
   ## What is the purpose of the change
   
   SQL: select *from t1 limit 1
   Now source will scan full table, if we can introduce LimitableTableSource, 
let source know the limit line, source can just read one row is OK.
   
   Adds support for limiting push-down to a TableSource. A TableSource 
extending this interface is able to limit the number of records.
   
   ## Verifying this change
   
   LimitTest for plan test.
   LimitITCase for it case.
   
   ## Does this pull request potentially affect one of the following parts:
   
     - Dependencies (does it add or upgrade a dependency): no
     - The public API, i.e., is any changed class annotated with 
`@Public(Evolving)`: no
     - The serializers: no
     - The runtime per-record code paths (performance sensitive): no
     - Anything that affects deployment or recovery: JobManager (and its 
components), Checkpointing, Yarn/Mesos, ZooKeeper: no
     - The S3 file system connector: no
   
   ## Documentation
   
     - Does this pull request introduce a new feature? yes
     - If yes, how is the feature documented? JavaDocs

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