trait SupportsPushDownAggregates extends ScanBuilder
A mix-in interface for ScanBuilder. Data sources can implement this interface to
push down aggregates.
If the data source can't fully complete the grouping work, then
#supportCompletePushDown(Aggregation) should return false, and Spark will group the data
source output again. For queries like "SELECT min(value) AS m FROM t GROUP BY key", after
pushing down the aggregate to the data source, the data source can still output data with
duplicated keys, which is OK as Spark will do GROUP BY key again. The final query plan can be
something like this:
Aggregate [key#1], [min(min_value#2) AS m#3]
+- RelationV2[key#1, min_value#2]
Similarly, if there is no grouping expression, the data source can still output more than one
rows.When pushing down operators, Spark pushes down filters to the data source first, then push down aggregates or apply column pruning. Depends on data source implementation, aggregates may or may not be able to be pushed down with filters. If pushed filters still need to be evaluated after scanning, aggregates can't be pushed down.
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- SupportsPushDownAggregates.java
- Since
3.2.0
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- abstract def build(): Scan
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- ScanBuilder
- abstract def pushAggregation(aggregation: Aggregation): Boolean
Pushes down Aggregation to datasource.
Pushes down Aggregation to datasource. The order of the datasource scan output columns should be: grouping columns, aggregate columns (in the same order as the aggregate functions in the given Aggregation).
- aggregation
Aggregation in SQL statement.
- returns
true if the aggregation can be pushed down to datasource, false otherwise.
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- def supportCompletePushDown(aggregation: Aggregation): Boolean
Whether the datasource support complete aggregation push-down.
Whether the datasource support complete aggregation push-down. Spark will do grouping again if this method returns false.
- aggregation
Aggregation in SQL statement.
- returns
true if the aggregation can be pushed down to datasource completely, false otherwise.
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(Since version 9)