Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/15544#discussion_r139188790
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/ApproxCountDistinctForIntervals.scala
---
@@ -0,0 +1,248 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.catalyst.expressions.aggregate
+
+import java.util
+
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.analysis.TypeCheckResult
+import
org.apache.spark.sql.catalyst.analysis.TypeCheckResult.{TypeCheckFailure,
TypeCheckSuccess}
+import org.apache.spark.sql.catalyst.expressions.{AttributeReference,
ExpectsInputTypes, Expression, ExpressionDescription}
+import org.apache.spark.sql.catalyst.util.{ArrayData, GenericArrayData,
HyperLogLogPlusPlusHelper}
+import org.apache.spark.sql.types._
+
+/**
+ * This function counts the approximate number of distinct values (ndv) in
+ * intervals constructed from endpoints specified in
`endpointsExpression`. The endpoints will be
+ * sorted into ascending order. To count ndv's in these intervals, apply
the HyperLogLogPlusPlus
+ * algorithm in each of them.
+ * @param child to estimate the ndv's of.
+ * @param endpointsExpression to construct the intervals.
+ * @param relativeSD The maximum estimation error allowed in the
HyperLogLogPlusPlus algorithm.
+ */
+@ExpressionDescription(
--- End diff --
we don't need this if it's an internal func
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