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_r237491775
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File path: docs/dev/table/streaming/match_recognize.md
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@@ -211,6 +211,66 @@ 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.
+### Scalar & Aggregate functions
+
+One can use scalar and aggregate functions in those clauses, both
[built-in]({{ site.baseurl }}/dev/table/sql.html#built-in-functions) as well as
provide [user defined]({{ site.baseurl }}/dev/table/udfs.html) functions.
+
+Aggregate functions are applied to subset of rows mapped to a match. To
understand how those subsets are evaluated have a look at the [event stream
navigation](#pattern-navigation) section.
+
+With a task to find the longest period of time for which the average price of
a ticker did not go below certain threshold, one can see 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 `A` pattern variable as long as
the average price of them does not exceed 15. Which will happen at `01-Apr-11
10:00:04`. The next such period that starts then will
+exceed average price of 15 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 %}
+
+An important thing to have in mind is how aggregates behave in situation when
no rows where mapped to certain pattern variable. Every aggregate, beside
`COUNT` will produce `null` in those cases. `COUNT` on the other hand will
Review comment:
```
An important thing to have in mind is how aggregates behave in situation
when no rows were mapped to a certain pattern variable. Every aggregate (except
for `COUNT`) will produce `null` in those cases. `COUNT` on the other hand will
produce 0.
```
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