Address review comments

Signed-off-by: Andy Kwok <[email protected]>

DEVELOPER_GUIDE.md

@@ -13,7 +13,7 @@ sbt test
 ```
 To run a specific unit test in SBT, use the testOnly command with the full path of the test class:
 ```
-sbt test:testOnly org.opensearch.flint.spark.ppl.PPLLogicalPlanTrendlineCommandTranslatorTestSuite
+sbt "; project pplSparkIntegration; test:testOnly org.opensearch.flint.spark.ppl.PPLLogicalPlanTrendlineCommandTranslatorTestSuite"
 ```
 
 
@@ -30,7 +30,7 @@ If you get integration test failures with error message "Previous attempts to fi
 
 Running only a selected set of integration test suites is possible with the following command:
 ```
-sbt "project integtest" it:testOnly org.opensearch.flint.spark.ppl.FlintSparkPPLTrendlineITSuite
+sbt "; project integtest; it:testOnly org.opensearch.flint.spark.ppl.FlintSparkPPLTrendlineITSuite"
 ```
 This command runs only the specified test suite within the integtest submodule.
 

ppl-spark-integration/src/main/java/org/opensearch/sql/expression/function/BuiltinFunctionName.java

@@ -282,8 +282,6 @@ public enum BuiltinFunctionName {
   MULTIMATCHQUERY(FunctionName.of("multimatchquery")),
   WILDCARDQUERY(FunctionName.of("wildcardquery")),
   WILDCARD_QUERY(FunctionName.of("wildcard_query")),
-
-  NTH_VALUE(FunctionName.of("nth_value")),
   COALESCE(FunctionName.of("coalesce"));
 
   private FunctionName name;

ppl-spark-integration/src/main/java/org/opensearch/sql/ppl/utils/TrendlineCatalystUtils.java

@@ -5,13 +5,15 @@
 
 package org.opensearch.sql.ppl.utils;
 
+import org.apache.spark.sql.catalyst.analysis.UnresolvedFunction;
 import org.apache.spark.sql.catalyst.expressions.*;
 import org.opensearch.sql.ast.expression.*;
 import org.opensearch.sql.ast.expression.Literal;
 import org.opensearch.sql.ast.tree.Trendline;
 import org.opensearch.sql.expression.function.BuiltinFunctionName;
 import org.opensearch.sql.ppl.CatalystExpressionVisitor;
 import org.opensearch.sql.ppl.CatalystPlanContext;
+import scala.collection.mutable.Seq;
 import scala.Option;
 import scala.Tuple2;
 
@@ -22,6 +24,8 @@
 import java.util.stream.Collectors;
 
 import static org.opensearch.sql.ppl.utils.DataTypeTransformer.seq;
+import static scala.Option.empty;
+import static scala.collection.JavaConverters.asScalaBufferConverter;
 
 public interface TrendlineCatalystUtils {
 
@@ -208,13 +212,14 @@ private static List<Expression> getNthValueAggregations(CatalystExpressionVisito
         for (int i = 1; i <= dataPoints; i++) {
             // Get the offset parameter
             Expression offSetExpression = parseIntToExpression(visitor, context, i);
-
-            // Composite the nth_value expression.
-            Function func =  new Function(BuiltinFunctionName.NTH_VALUE.name(),
-                    List.of(node.getDataField(), new Literal(i, DataType.INTEGER)));
-
-            visitor.visitFunction(func, context);
-            Expression nthValueExp = context.popNamedParseExpressions().get();
+            // Get the dataField in Expression
+            visitor.analyze(node.getDataField(), context);
+            Expression dataField = context.popNamedParseExpressions().get();
+            // nth_value Expression
+            UnresolvedFunction nthValueExp = new UnresolvedFunction(
+                    asScalaBufferConverter(List.of("nth_value")).asScala().seq(),
+                    asScalaBufferConverter(List.of(dataField, offSetExpression)).asScala().seq(),
+                    false, empty(), false);
 
             expressions.add(new Multiply(
                     new WindowExpression(nthValueExp, windowDefinition), offSetExpression));