hi、 请问下面这两种输出流实现的时候，怎么确定选用实现upsert还是retract，因为看这两个api描述功能是差不多的，并且代码实现只是upsert多了两个方法，具体完成的功能貌似可以是一样的，这要怎么作出选择：
public interface RetractStreamTableSink<T> extends StreamTableSink <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/table/sinks/StreamTableSink.html><Tuple2 <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/api/java/tuple/Tuple2.html><Boolean <http://docs.oracle.com/javase/7/docs/api/java/lang/Boolean.html?is-external=true>,T>> Defines an external TableSink <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/table/sinks/TableSink.html> to emit a streaming Table <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/table/api/Table.html> with insert, update, and delete changes. The table will be converted into a stream of accumulate and retraction messages which are encoded as Tuple2 <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/api/java/tuple/Tuple2.html>. The first field is a Boolean <http://docs.oracle.com/javase/7/docs/api/java/lang/Boolean.html?is-external=true> flag to indicate the message type. The second field holds the record of the requested type T. A message with true Boolean <http://docs.oracle.com/javase/7/docs/api/java/lang/Boolean.html?is-external=true> flag is an accumulate (or add) message. A message with false flag is a retract message. public interface UpsertStreamTableSink<T> extends StreamTableSink <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/table/sinks/StreamTableSink.html><Tuple2 <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/api/java/tuple/Tuple2.html><Boolean <http://docs.oracle.com/javase/7/docs/api/java/lang/Boolean.html?is-external=true>,T>> Defines an external TableSink <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/table/sinks/TableSink.html> to emit a streaming Table <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/table/api/Table.html> with insert, update, and delete changes. The Table <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/table/api/Table.html> must be have unique key fields (atomic or composite) or be append-only. If the Table <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/table/api/Table.html> does not have a unique key and is not append-only, a TableException <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/table/api/TableException.html> will be thrown. The unique key of the table is configured by the setKeyFields(String[]) <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/table/sinks/UpsertStreamTableSink.html#setKeyFields-java.lang.String:A-> method. The Table <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/table/api/Table.html> will be converted into a stream of upsert and delete messages which are encoded as Tuple2 <https://ci.apache.org/projects/flink/flink-docs-release-1.11/api/java/org/apache/flink/api/java/tuple/Tuple2.html>. The first field is a Boolean <http://docs.oracle.com/javase/7/docs/api/java/lang/Boolean.html?is-external=true> flag to indicate the message type. The second field holds the record of the requested type T. A message with true Boolean <http://docs.oracle.com/javase/7/docs/api/java/lang/Boolean.html?is-external=true> field is an upsert message for the configured key. A message with false flag is a delete message for the configured key. If the table is append-only, all messages will have a true flag and must be interpreted as insertions.
