[ 
https://issues.apache.org/jira/browse/FLINK-4469?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15711944#comment-15711944
 ] 

ASF GitHub Bot commented on FLINK-4469:
---------------------------------------

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

    https://github.com/apache/flink/pull/2653#discussion_r90052039
  
    --- Diff: 
flink-libraries/flink-table/src/main/scala/org/apache/flink/api/table/codegen/calls/TableFunctionCallGen.scala
 ---
    @@ -0,0 +1,78 @@
    +/*
    + * 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.flink.api.table.codegen.calls
    +
    +import org.apache.flink.api.common.typeinfo.TypeInformation
    +import org.apache.flink.api.table.codegen.CodeGenUtils._
    +import org.apache.flink.api.table.codegen.{CodeGenException, 
CodeGenerator, GeneratedExpression}
    +import org.apache.flink.api.table.functions.TableFunction
    +import 
org.apache.flink.api.table.functions.utils.UserDefinedFunctionUtils._
    +
    +/**
    +  * Generates a call to user-defined [[TableFunction]].
    +  *
    +  * @param tableFunction user-defined [[TableFunction]] that might be 
overloaded
    +  * @param signature actual signature with which the function is called
    +  * @param returnType actual return type required by the surrounding
    +  */
    +class TableFunctionCallGen(
    +    tableFunction: TableFunction[_],
    +    signature: Seq[TypeInformation[_]],
    +    returnType: TypeInformation[_])
    +  extends CallGenerator {
    +
    +  override def generate(
    +      codeGenerator: CodeGenerator,
    +      operands: Seq[GeneratedExpression])
    +    : GeneratedExpression = {
    +    // determine function signature
    +    val matchingSignature = getSignature(tableFunction.getClass, signature)
    +      .getOrElse(throw new CodeGenException("No matching signature 
found."))
    +
    +    // convert parameters for function (output boxing)
    +    val parameters = matchingSignature
    +        .zip(operands)
    +        .map { case (paramClass, operandExpr) =>
    +          if (paramClass.isPrimitive) {
    +            operandExpr
    +          } else {
    +            val boxedTypeTerm = 
boxedTypeTermForTypeInfo(operandExpr.resultType)
    +            val boxedExpr = 
codeGenerator.generateOutputFieldBoxing(operandExpr)
    +            val exprOrNull: String = if (codeGenerator.nullCheck) {
    +              s"${boxedExpr.nullTerm} ? null : ($boxedTypeTerm) 
${boxedExpr.resultTerm}"
    +            } else {
    +              boxedExpr.resultTerm
    +            }
    +            boxedExpr.copy(resultTerm = exprOrNull)
    +          }
    +        }
    +
    +    // generate function call
    +    val functionReference = 
codeGenerator.addReusableInstance(tableFunction)
    +    val functionCallCode =
    +      s"""
    +        |${parameters.map(_.code).mkString("\n")}
    +        |$functionReference.clear();
    +        
|$functionReference.eval(${parameters.map(_.resultTerm).mkString(", ")});
    +        |""".stripMargin
    +
    +    // has no result
    +    GeneratedExpression(functionReference, "false", functionCallCode, 
returnType)
    --- End diff --
    
    Btw. I created the constant `GeneratedExpression.NEVER_NULL` instead of 
`"false"` to make it more readable.


> Add support for user defined table function in Table API & SQL
> --------------------------------------------------------------
>
>                 Key: FLINK-4469
>                 URL: https://issues.apache.org/jira/browse/FLINK-4469
>             Project: Flink
>          Issue Type: New Feature
>          Components: Table API & SQL
>            Reporter: Jark Wu
>            Assignee: Jark Wu
>
> Normal user-defined functions, such as concat(), take in a single input row 
> and output a single output row. In contrast, table-generating functions 
> transform a single input row to multiple output rows. It is very useful in 
> some cases, such as look up in HBase by rowkey and return one or more rows.
> Adding a user defined table function should:
> 1. inherit from UDTF class with specific generic type T
> 2. define one or more evel function. 
> NOTE: 
> 1. the eval method must be public and non-static.
> 2. the generic type T is the row type returned by table function. Because of 
> Java type erasure, we can’t extract T from the Iterable.
> 3. use {{collect(T)}} to emit table row
> 4. eval method can be overload. Blink will choose the best match eval method 
> to call according to parameter types and number.
> {code}
> public class Word {
>   public String word;
>   public Integer length;
> }
> public class SplitStringUDTF extends UDTF<Word> {
>     public Iterable<Word> eval(String str) {
>         if (str != null) {
>             for (String s : str.split(",")) {
>                 collect(new Word(s, s.length()));
>             }
>         }
>     }
> }
> // in SQL
> tableEnv.registerFunction("split", new SplitStringUDTF())
> tableEnv.sql("SELECT a, b, t.* FROM MyTable, LATERAL TABLE(split(c)) AS 
> t(w,l)")
> // in Java Table API
> tableEnv.registerFunction("split", new SplitStringUDTF())
> // rename split table columns to “w” and “l”
> table.crossApply("split(c) as (w, l)")        
>      .select("a, b, w, l")
> // without renaming, we will use the origin field names in the POJO/case/...
> table.crossApply("split(c)")
>      .select("a, b, word, length")
> // in Scala Table API
> val split = new SplitStringUDTF()
> table.crossApply(split('c) as ('w, 'l))
>      .select('a, 'b, 'w, 'l)
> // outerApply for outer join to a UDTF
> table.outerApply(split('c))
>      .select('a, 'b, 'word, 'length)
> {code}
> See [1] for more information about UDTF design.
> [1] 
> https://docs.google.com/document/d/15iVc1781dxYWm3loVQlESYvMAxEzbbuVFPZWBYuY1Ek/edit#



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