twalthr commented on a change in pull request #7177: [FLINK-7599] [table]
Support for aggregates in MATCH_RECOGNIZE
URL: https://github.com/apache/flink/pull/7177#discussion_r241742837
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File path: docs/dev/table/streaming/match_recognize.md
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@@ -211,6 +211,65 @@ If a condition is not defined for a pattern variable, a
default condition will b
For a more detailed explanation about expressions that can be used in those
clauses, please have a look at the [event stream
navigation](#pattern-navigation) section.
+### Aggregations
+
+Aggregations can be used in `DEFINE` and `MEASURES` clauses. Both
[built-in]({{ site.baseurl }}/dev/table/sql.html#built-in-functions) and custom
[user defined]({{ site.baseurl }}/dev/table/udfs.html) functions are supported.
+
+Aggregate functions are applied to each subset of rows mapped to a match. In
order to understand how those subsets are evaluated have a look at the [event
stream navigation](#pattern-navigation) section.
+
+The task of the following example is to find the longest period of time for
which the average price of a ticker did not go below certain threshold. It
shows how expressible `MATCH_RECOGNIZE` can become with aggregations.
+This task can be performed with the following query:
+
+{% highlight sql %}
+SELECT *
+FROM Ticker
+MATCH_RECOGNIZE (
+ PARTITION BY symbol
+ ORDER BY rowtime
+ MEASURES
+ FIRST(A.rowtime) AS start_tstamp,
+ LAST(A.rowtime) AS end_tstamp,
+ AVG(A.price) AS avgPrice
+ ONE ROW PER MATCH
+ AFTER MATCH SKIP TO FIRST B
+ PATTERN (A+ B)
+ DEFINE
+ A AS AVG(A.price) < 15
+ ) MR;
+{% endhighlight %}
+
+Given this query and following input values:
+
+{% highlight text %}
+symbol rowtime price tax
+====== ==================== ======= =======
+'ACME' '01-Apr-11 10:00:00' 12 1
+'ACME' '01-Apr-11 10:00:01' 17 2
+'ACME' '01-Apr-11 10:00:02' 13 1
+'ACME' '01-Apr-11 10:00:03' 16 3
+'ACME' '01-Apr-11 10:00:04' 25 2
+'ACME' '01-Apr-11 10:00:05' 2 1
+'ACME' '01-Apr-11 10:00:06' 4 1
+'ACME' '01-Apr-11 10:00:07' 10 2
+'ACME' '01-Apr-11 10:00:08' 15 2
+'ACME' '01-Apr-11 10:00:09' 25 2
+'ACME' '01-Apr-11 10:00:10' 30 1
+{% endhighlight %}
+
+The query will accumulate events as part of the pattern variable `A` as long
as the average price of them does not exceed `15`. For example, such a limit
exceeding happens at `01-Apr-11 10:00:04`.
+The following period exceeds the average price of `15` again at `01-Apr-11
10:00:10`. Thus the results for said query will be:
+
+{% highlight text %}
+ symbol start_tstamp end_tstamp avgPrice
+========= ================== ================== ============
+ACME 01-APR-11 10:00:00 01-APR-11 10:00:03 14.5
+ACME 01-APR-11 10:00:04 01-APR-11 10:00:09 13.5
+{% endhighlight %}
+
+<span class="label label-info">Note</span> Aggregations can be applied to
expressions, but only if they reference a single pattern variable. Thus
`SUM(A.price * A.tax)` is a valid one, but `AVG(A.price * B.tax)` is not.
+
+<span class="label label-danger">Attention</span> `DISTINCT` aggregations are
not supported.
Review comment:
Can you open a follow up issue for this if it does not exist yet?
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