MartijnVisser commented on a change in pull request #18316:
URL: https://github.com/apache/flink/pull/18316#discussion_r781188418
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File path: docs/content.zh/docs/dev/table/config.md
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@@ -91,6 +91,39 @@ Flink SQL> SET 'table.exec.mini-batch.size' = '5000';
{{< /tab >}}
{{< /tabs >}}
+### Idle State Retention Time
+
+Many queries aggregate or join records on one or more key attributes. When
such a query is executed
+on a stream, the continuous query needs to collect records or maintain partial
results per key.
+If the key domain of the input stream is evolving, i.e., the active key values
are changing over
+time, the continuous query accumulates more and more state as more and more
distinct keys are
+observed. However, often keys become inactive after some time and their
corresponding state
+becomes stale and useless.
+
+For example the following query computes the number of clicks per session.
+
+```sql
+SELECT sessionId, COUNT(*) FROM clicks GROUP BY sessionId;
+```
+
+The `sessionId` attribute is used as a grouping key and the continuous query
maintains a count
+for each `sessionId` it observes. The `sessionId` attribute is evolving over
time and `sessionId`
+values are only active until the session ends, i.e., for a limited period of
time. However, the
+continuous query cannot know about this property of `sessionId` and expects
that every `sessionId`
+value can occur at any point of time. It maintains a count for each observed
`sessionId` value.
+Consequently, the total state size of the query is continuously growing as
more and more `sessionId`
+values are observed.
+
+The *Idle State Retention Time* parameter [`table.exec.state.ttl`]({{< ref
"docs/dev/table/config" >}}#table-exec-state-ttl)
+define for how long the state of a key is retained without being updated
before it is removed.
+For the previous example query, the count of a`sessionId` would be removed as
soon as it has not
+been updated for the configured period of time.
+
+By removing the state of a key, the continuous query completely forgets that
it has seen this key
+before. If a record with a key, whose state has been removed before, is
processed, the record will
+be treated as if it was the first record with the respective key. For the
example above this means
+that the count of a `sessionId` would start again at `0`.
+
Review comment:
@twalthr This is the re-introduction of the documentation that was
removed, to make sure that the relevant part of 'Query Configuration' now has a
good documentation spot again and all broken links point to this documentation
again.
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