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Subject [GitHub] twalthr commented on a change in pull request #7177: [FLINK-7599] [table] Support for aggregates in MATCH_RECOGNIZE
Date Fri, 14 Dec 2018 15:17:31 GMT
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_r241787811
 
 

 ##########
 File path: docs/dev/table/streaming/match_recognize.md
 ##########
 @@ -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;
 
 Review comment:
   @dawidwys what is `MR` actually doing? Do we have to call `SELECT MR.start_tstamp`?

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