Benjamin Lerer created CASSANDRA-17222:
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Summary: Add new metrics to track the number of requests performed
by GROUP BY and Aggregation queries
Key: CASSANDRA-17222
URL: https://issues.apache.org/jira/browse/CASSANDRA-17222
Project: Cassandra
Issue Type: Improvement
Components: Observability/Metrics
Reporter: Benjamin Lerer
When a user perform a GROUP BY query or an aggregate query (e.g. {{SELECT
count(*) FROM my_table}}) internally C* will send multiple internal requests to
avoid running out of memory. The page size used for those internal queries is
the same as the external page size.
Having a some visibility on the number of internal requests happening for a
group by or an aggregate query is important as it might help administrators to
debug performance issues.
We should add some separate metrics for GROUP BY queries and Aggregate queries
+Additional information for newcomers:+
* A new metric class called {{AggregationMetrics}} should be created with an
{{Histogram}} called {{internalPagesPerGroupByQuerie}} and another called
{{internalPagesPerAggregateQuerie}} (see {{BatchMetrics}} for an example
* High level query paging are managed by {{AggregationQueryPager}}. The number
of queries performed should be incremented within {{fetchSubPage}} and the
metrics should be updated on close.
* To test that the numbers are reliable, you need to create a new Unit Test
{{AggregationMetricsTest}}. To have some example of how to test group by
queries with paging, you can look into
{{SelectGroupByTest.testGroupByWithPaging()}} to check how to clear the
histograms between test you can look into
{{BatchMetricsTests.clearHistogram()}}
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