Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/3166#discussion_r101316851
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/plan/rules/util/RexProgramExpressionExtractor.scala
---
@@ -0,0 +1,198 @@
+/*
+ * 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.table.plan.rules.util
+
+import java.util
+
+import org.apache.calcite.rel.core.TableScan
+import org.apache.calcite.rex._
+import org.apache.calcite.sql.{SqlKind, SqlOperator}
+import org.apache.calcite.tools.RelBuilder
+import org.apache.flink.table.api.TableEnvironment
+import org.apache.flink.table.expressions._
+import org.apache.flink.table.sources.TableSource
+
+import scala.collection.JavaConversions._
+import scala.collection.JavaConverters._
+import scala.collection.mutable
+
+object RexProgramExpressionExtractor {
+
+ /**
+ * converts a rexProgram condition into expression
+ *
+ * @param rexProgram The RexProgram to analyze
+ * @return converted expression
+ */
+ def extractExpression(rexProgram: RexProgram): Expression = {
+
+ val refInputToName = getInputsWithNames(rexProgram)
+ val visitor = new ExpressionVisitor(refInputToName)
+
+ val condition = rexProgram.getCondition
+ if (condition == null) {
+ return null
+ }
+
+ rexProgram.expandLocalRef(condition).accept(visitor)
+ val parsedExpression =
ExpressionParser.parseExpression(visitor.getStringPredicate)
--- End diff --
Converting by generating and parsing strings is not very reliable.
We should rather map `RexNode` directly to `Expression`.
@twalthr suggested to add the translation logic to the corresponding
`Expression` next to the `toRexNode()` method. We would need to find a
"dictionary" to identify the relevant `Expression` for a `RexNode` though.
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