Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/13494#discussion_r68375446
--- Diff:
sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveMetastoreCatalog.scala ---
@@ -457,6 +458,125 @@ private[hive] class
HiveMetastoreCatalog(sparkSession: SparkSession) extends Log
allowExisting)
}
}
+
+ /**
+ * When scanning only partition columns, get results based on metadata
without scanning files.
+ * It is used for distinct or distinct/Max/Min aggregations, example:
max(partition).
+ */
+ object MetadataOnlyOptimizer extends Rule[LogicalPlan] {
+
+ private def canSupportMetadataOnly(a: Aggregate): Boolean = {
+ val aggregateExpressions = a.aggregateExpressions.flatMap { expr =>
+ expr.collect {
+ case agg: AggregateExpression => agg
+ }
+ }.distinct
+ aggregateExpressions.forall { agg =>
--- End diff --
can we add more comments to explain the condition to enable metadata
optimization? e.g. the agg expression must only reference to partition columns,
all distinct agg functions, `Max` and `Min`, etc.
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