beyond1920 commented on a change in pull request #16963:
URL: https://github.com/apache/flink/pull/16963#discussion_r698156109
##########
File path: docs/content/docs/dev/table/sql/queries/window-tvf.md
##########
@@ -282,4 +282,64 @@ Here is an example invocation on the Bid table:
+------------------+------------------+-------+
```
+### SESSION
+
+The session windows group elements by sessions of activity. Session windows do
not overlap and do not have a fixed start and end time, in contrast to `TUMBLE`
windows and `HOP` windows. Instead a session window closes when it does not
receive elements for a certain period of time, i.e., when a gap of inactivity
occurred.
+
+The `SESSION` function specifies session gap which defines how long the period
of inactivity is. When this period expires, the current session closes and
subsequent elements are assigned to a new session window.
+
+{{< img src="/fig/session-windows.svg" alt="Session Windows" width="70%">}}
+
+The `SESSION` functions assign windows based on a [time attribute]({{< ref
"docs/dev/table/concepts/time_attributes" >}}) column. The return value of
`SESSION` is a new relation that includes all columns of original relation as
well as additional 3 columns named "window_start", "window_end", "window_time"
to indicate the assigned window. The original time attribute "timecol" will be
a regular timestamp column after window TVF.
+
+`SESSION` takes three required parameters and one optional parameter.
+
+```sql
+SESSION(TABLE data, DESCRIPTOR(timecol), gap)
+```
+
+- `data`: is a table parameter that can be any relation with an time attribute
column.
+- `timecol`: is a column descriptor indicating which [time attributes]({{< ref
"docs/dev/table/concepts/time_attributes" >}}) column of data should be mapped
to session windows.
+- `KEY`: is an optional parameter. If does not specify `key`, Session window
is applied for all input records, otherwise, Session window is applied per key.
+- `gap`: is a duration specifying how long the period of inactivity is. When
this period expires, the current session closes and subsequent elements are
assigned to a new session window.
+
+Here is an example invocation on the Bid table:
+
+```sql
+-- NOTE: Currently Flink doesn't support evaluating individual window
table-valued function,
+-- window table-valued function should be used with aggregate operation,
+-- this example is just used for explaining the syntax and the data produced
by table-valued function.
+> SELECT * FROM TABLE(
+ SESSION(TABLE Bid, DESCRIPTOR(bidtime), INTERVAL '3' MINUTES));
+-- or with the named params
+-- note: the DATA param must be the first
+> SELECT * FROM TABLE(
+ SESSION(
+ DATA => TABLE Bid,
+ TIMECOL => DESCRIPTOR(bidtime),
+ GAP => INTERVAL '3' MINUTES));
++------------------+-------+------+------------------+------------------+-------------------------+
+| bidtime | price | item | window_start | window_end |
window_time |
++------------------+-------+------+------------------+------------------+-------------------------+
+| 2020-04-15 08:05 | 4.00 | C | 2020-04-15 08:05 | 2020-04-15 08:16 |
2020-04-15 08:15:59.999 |
+| 2020-04-15 08:07 | 2.00 | A | 2020-04-15 08:05 | 2020-04-15 08:16 |
2020-04-15 08:15:59.999 |
+| 2020-04-15 08:09 | 5.00 | D | 2020-04-15 08:05 | 2020-04-15 08:16 |
2020-04-15 08:15:59.999 |
+| 2020-04-15 08:11 | 3.00 | B | 2020-04-15 08:05 | 2020-04-15 08:16 |
2020-04-15 08:15:59.999 |
+| 2020-04-15 08:13 | 1.00 | E | 2020-04-15 08:05 | 2020-04-15 08:16 |
2020-04-15 08:15:59.999 |
+| 2020-04-15 08:17 | 6.00 | F | 2020-04-15 08:17 | 2020-04-15 08:20 |
2020-04-15 08:19:59.999 |
++------------------+-------+------+------------------+------------------+-------------------------+
+
+-- apply aggregation on the session windowed table
+> SELECT window_start, window_end, SUM(price)
+ FROM TABLE(
+ SESSION(TABLE Bid, DESCRIPTOR(bidtime), INTERVAL '3' MINUTES))
Review comment:
done
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