zhangqingru created FLINK-20345:
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Summary: Adds an Expand node only when there are more then one distinct aggregate function in an Aggregate when executes SplitAggregateRule
Key: FLINK-20345
URL:
https://issues.apache.org/jira/browse/FLINK-20345 Project: Flink
Issue Type: Improvement
Components: Table SQL / Planner
Affects Versions: 1.11.2
Reporter: zhangqingru
Fix For: 1.11.3
As mentioned in [Flink Document|
https://ci.apache.org/projects/flink/flink-docs-stable/dev/table/tuning/streaming_aggregation_optimization.html], we could split distinct aggregation to solve skew data on distinct keys which is a very good optimization. However, an unnecessary `Expand` node will be generated under some special cases, like the following sql.
{code:java}
SELECT COUNT(c) AS pv, COUNT(DISTINCT c) AS uv FROM T GROUP BY a
{code}
Which plan is like the following text, the Expand and filter condition in aggregate functions could be removed.
{code:java}
Sink(name=[DataStreamTableSink], fields=[pv, uv])
+- Calc(select=[pv, uv])
+- GroupAggregate(groupBy=[a], partialFinalType=[FINAL], select=[a, $SUM0_RETRACT($f2_0) AS $f1, $SUM0_RETRACT($f3) AS $f2])
+- Exchange(distribution=[hash[a]])
+- GroupAggregate(groupBy=[a, $f2], partialFinalType=[PARTIAL], select=[a, $f2, COUNT(c) FILTER $g_1 AS $f2_0, COUNT(DISTINCT c) FILTER $g_0 AS $f3])
+- Exchange(distribution=[hash[a, $f2]])
+- Calc(select=[a, c, $f2, =($e, 1) AS $g_1, =($e, 0) AS $g_0])
+- Expand(projects=[{a=[$0], c=[$1], $f2=[$2], $e=[0]}, {a=[$0], c=[$1], $f2=[null], $e=[1]}])
+- Calc(select=[a, c, MOD(HASH_CODE(c), 1024) AS $f2])
+- MiniBatchAssigner(interval=[1000ms], mode=[ProcTime])
+- DataStreamScan(table=[[default_catalog, default_database, T]], fields=[a, b, c]){code}
An `Expand` node only is necessary when multiple aggregate function with different distinct keys appears in one Aggregate.
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