Update implementation of NumberToStringFunction

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

core/src/main/java/org/opensearch/sql/calcite/ExtendedRexBuilder.java

@@ -153,7 +153,7 @@ public RexNode makeCast(
     // Use a custom operator when casting an approximate numeric (e.g. double) to a character type.
     // This patch is necessary because Calcite's built-in CAST converts 0.0 to 0E0 as string.
     else if (SqlTypeUtil.isApproximateNumeric(exp.getType()) && SqlTypeUtil.isCharacter(type)) {
-      // NUMBER_TO_STRING first box the number, then invoke its toString method
+      // NUMBER_TO_STRING uses java's built-in method to get the string representation of a number
       return makeCall(type, PPLBuiltinOperators.NUMBER_TO_STRING, List.of(exp));
     }
     return super.makeCast(pos, type, exp, matchNullability, safe, format);

core/src/main/java/org/opensearch/sql/expression/function/udf/math/NumberToStringFunction.java

@@ -6,11 +6,13 @@
 package org.opensearch.sql.expression.function.udf.math;
 
 import java.util.List;
+import java.util.Objects;
 import org.apache.calcite.adapter.enumerable.NotNullImplementor;
 import org.apache.calcite.adapter.enumerable.NullPolicy;
 import org.apache.calcite.adapter.enumerable.RexToLixTranslator;
 import org.apache.calcite.linq4j.tree.Expression;
 import org.apache.calcite.linq4j.tree.Expressions;
+import org.apache.calcite.linq4j.tree.Primitive;
 import org.apache.calcite.rex.RexCall;
 import org.apache.calcite.sql.type.SqlReturnTypeInference;
 import org.opensearch.sql.calcite.utils.PPLOperandTypes;
@@ -45,7 +47,9 @@ public static class NumberToStringImplementor implements NotNullImplementor {
     public Expression implement(
         RexToLixTranslator translator, RexCall call, List<Expression> translatedOperands) {
       Expression operand = translatedOperands.get(0);
-      return Expressions.call(Expressions.box(operand), "toString");
+      Primitive primitive = Primitive.of(operand.getType());
+      Objects.requireNonNull(primitive);
+      return Expressions.call(primitive.getBoxClass(), "toString", operand);
     }
   }
 }