Github user wzhfy commented on a diff in the pull request:

    https://github.com/apache/spark/pull/15443#discussion_r83405687
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/StringHistogram.scala
 ---
    @@ -0,0 +1,209 @@
    +/*
    + * 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.nio.ByteBuffer
    +
    +import scala.collection.immutable.TreeMap
    +import scala.collection.mutable
    +
    +import com.google.common.primitives.{Ints, Longs}
    +
    +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.{Expression, 
ExpressionDescription}
    +import 
org.apache.spark.sql.catalyst.expressions.aggregate.StringHistogram.StringHistogramInfo
    +import org.apache.spark.sql.catalyst.util.ArrayBasedMapData
    +import org.apache.spark.sql.types._
    +import org.apache.spark.unsafe.types.UTF8String
    +
    +/**
    + * The StringHistogram function returns the histogram bins - (distinct 
value, frequency) pairs
    + * for a string column.
    + * @param child child expression that can produce column value with 
`child.eval(inputRow)`
    + * @param numBinsExpression The maximum number of bins. When the number of 
distinct values is
    + *                          larger than this, the function will return an 
empty result.
    + */
    +@ExpressionDescription(
    +  usage = "_FUNC_(col, numBins) - Returns histogram bins with the maximum 
number of bins allowed.")
    +case class StringHistogram(
    +    child: Expression,
    +    numBinsExpression: Expression,
    +    override val mutableAggBufferOffset: Int,
    +    override val inputAggBufferOffset: Int) extends 
TypedImperativeAggregate[StringHistogramInfo] {
    +
    +  def this(child: Expression, numBinsExpression: Expression) = {
    +    this(child, numBinsExpression, 0, 0)
    +  }
    +
    +  // Mark as lazy so that numBinsExpression is not evaluated during tree 
transformation.
    +  private lazy val numBins: Int = 
numBinsExpression.eval().asInstanceOf[Int]
    +
    +  override def checkInputDataTypes(): TypeCheckResult = {
    +    val defaultCheck = super.checkInputDataTypes()
    +    if (defaultCheck.isFailure) {
    +      defaultCheck
    +    } else if (!numBinsExpression.foldable) {
    +      TypeCheckFailure(s"The maximum number of bins provided must be a 
constant literal")
    --- End diff --
    
    remove "s"


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