Github user alpinegizmo commented on a diff in the pull request:
https://github.com/apache/flink/pull/4020#discussion_r119151823
--- Diff: docs/dev/table/streaming.md ---
@@ -39,17 +39,293 @@ Dynamic table
Time Attributes
---------------
-### Event-time
+Flink supports different notions of *time* in streaming programs.
-* DataStream: Timestamps & WMs required, `.rowtime` (replace attribute or
extend schema)
-* TableSource: Timestamps & WMs & DefinedRowtimeAttribute
+- *Processing time* refers to the system time of the machine (also known
as "wall-clock time") that is executing the respective operation.
+- *Event time* is the time that each individual event occurred on its
producing device.
+- *Ingestion time* is the time that events enter Flink, internally, it is
treated similar to event time.
-{% top %}
+For more information about time handling in Flink, see the introduction
about [Event Time and Watermarks]({{ site.baseurl }}/dev/event_time.html).
+
+Table programs assume that a corresponding time characteristic has been
specified for the streaming environment:
+
+<div class="codetabs" markdown="1">
+<div data-lang="java" markdown="1">
+{% highlight java %}
+final StreamExecutionEnvironment env =
StreamExecutionEnvironment.getExecutionEnvironment();
+
+env.setStreamTimeCharacteristic(TimeCharacteristic.ProcessingTime); //
default
+
+// alternatively:
+// env.setStreamTimeCharacteristic(TimeCharacteristic.IngestionTime);
+// env.setStreamTimeCharacteristic(TimeCharacteristic.EventTime);
+{% endhighlight %}
+</div>
+<div data-lang="scala" markdown="1">
+{% highlight scala %}
+val env = StreamExecutionEnvironment.getExecutionEnvironment
+
+env.setStreamTimeCharacteristic(TimeCharacteristic.ProcessingTime)
//default
+
+// alternatively:
+// env.setStreamTimeCharacteristic(TimeCharacteristic.IngestionTime)
+// env.setStreamTimeCharacteristic(TimeCharacteristic.EventTime)
+{% endhighlight %}
+</div>
+</div>
+
+Time-based operations such as [windows]({{ site.baseurl
}}/dev/table/tableApi.html) in both the Table API and SQL require information
about the notion of time and its origin. Therefore, tables can offer *logical
time attributes* for indicating time and accessing corresponding timestamps in
table programs.
+
+Time attributes can be part of every table schema. They are defined when
creating a table from a `DataStream` or pre-defined when using a `TableSource`.
Once a time attribute is defined at the beginning, it can be referenced as
field and used in time-based operations.
+
+As long as a time attribute is not modified and simply forwarded from one
part of the query to another, it remains a valid time attribute. Time
attributes behave like regular timestamps and can be accessed for calculations.
If a time attribute is used in a calculation, it will be materialized and
becomes a regular timestamp. Regular timestamps do not cooperate with Flink's
time and watermarking system and can thus not be used for time-based operations
anymore.
--- End diff --
As long as a time attribute is not modified and is simply forwarded ...
... and thus can not be used for further time-based operations.
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