Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/5016#discussion_r151095005
--- Diff: docs/dev/table/sourceSinks.md ---
@@ -316,7 +316,7 @@ A table schema field of type `SQL_TIMESTAMP` can be
declared as rowtime attribut
* the name of the field,
* a `TimestampExtractor` that computes the actual value for the attribute
(usually from one or more other attributes), and
-* a `WatermarkStrategy` that specifies how watermarks are generated for
the the rowtime attribute.
+* a `WatermarkStrategy` that specifies how watermarks are generated for
the the rowtime attribute (With the default value `null` to indicate no
watermark should be generated).
--- End diff --
I think we should not use `null` to indicate that no watermarks should be
assigned.
Rather provide a `WatermarkStrategy` that indicates that existing
watermarks are preserved like:
```
class PreserveWatermarks extends WatermarkStrategy {
def INSTANCE: PreserveWatermarks = new PreserveWatermarks
}
```
---
