Fix DOUBLE to STRING cast rendering zero values in scientific notation

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

@@ -150,6 +150,12 @@ public RexNode makeCast(
             String.format(Locale.ROOT, "Cannot cast from %s to %s", argExprType, udt.name()));
       };
     }
+    // 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
+      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/PPLBuiltinOperators.java

@@ -78,6 +78,7 @@
 import org.opensearch.sql.expression.function.udf.math.DivideFunction;
 import org.opensearch.sql.expression.function.udf.math.EulerFunction;
 import org.opensearch.sql.expression.function.udf.math.ModFunction;
+import org.opensearch.sql.expression.function.udf.math.NumberToStringFunction;
 
 /** Defines functions and operators that are implemented only by PPL */
 public class PPLBuiltinOperators extends ReflectiveSqlOperatorTable {
@@ -355,6 +356,8 @@ public class PPLBuiltinOperators extends ReflectiveSqlOperatorTable {
       RELEVANCE_QUERY_FUNCTION_INSTANCE.toUDF("query_string", false);
   public static final SqlOperator MULTI_MATCH =
       RELEVANCE_QUERY_FUNCTION_INSTANCE.toUDF("multi_match", false);
+  public static final SqlOperator NUMBER_TO_STRING =
+      new NumberToStringFunction().toUDF("NUMBER_TO_STRING");
 
   /**
    * Returns the PPL specific operator table, creating it if necessary.

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

@@ -0,0 +1,51 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.expression.function.udf.math;
+
+import java.util.List;
+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.rex.RexCall;
+import org.apache.calcite.sql.type.SqlReturnTypeInference;
+import org.opensearch.sql.calcite.utils.PPLOperandTypes;
+import org.opensearch.sql.calcite.utils.PPLReturnTypes;
+import org.opensearch.sql.expression.function.ImplementorUDF;
+import org.opensearch.sql.expression.function.UDFOperandMetadata;
+
+/**
+ * A custom implementation of number to string cast.
+ *
+ * <p>This operator is necessary because Calcite's built-in CAST converts 0.0 to 0E0 when casting it
+ * to string.
+ */
+public class NumberToStringFunction extends ImplementorUDF {
+  public NumberToStringFunction() {
+    super(new NumberToStringImplementor(), NullPolicy.ANY);
+  }
+
+  @Override
+  public SqlReturnTypeInference getReturnTypeInference() {
+    return PPLReturnTypes.STRING_FORCE_NULLABLE;
+  }
+
+  @Override
+  public UDFOperandMetadata getOperandMetadata() {
+    return PPLOperandTypes.NUMERIC;
+  }
+
+  public static class NumberToStringImplementor implements NotNullImplementor {
+
+    @Override
+    public Expression implement(
+        RexToLixTranslator translator, RexCall call, List<Expression> translatedOperands) {
+      Expression operand = translatedOperands.get(0);
+      return Expressions.call(Expressions.box(operand), "toString");
+    }
+  }
+}

integ-test/src/test/java/org/opensearch/sql/ppl/CastFunctionIT.java

@@ -18,6 +18,7 @@
 import static org.opensearch.sql.util.MatcherUtils.verifySchema;
 
 import java.io.IOException;
+import java.util.Locale;
 import org.json.JSONObject;
 import org.junit.Test;
 import org.opensearch.sql.common.antlr.SyntaxCheckException;
@@ -432,4 +433,17 @@ public void testCastToIP() throws IOException {
         "IP address string 'invalid_ip' is not valid. Error details: invalid_ip IP Address error:"
             + " validation options do not allow you to specify a non-segmented single value");
   }
+
+  @Test
+  public void testCastDoubleAsString() throws IOException {
+    JSONObject actual =
+        executeQuery(
+            String.format(
+                Locale.ROOT,
+                "source=%s | head 1 | eval d = cast(0 as double) | eval s = cast(d as string) |"
+                    + " fields s",
+                TEST_INDEX_STATE_COUNTRY));
+    verifySchema(actual, schema("s", "string"));
+    verifyDataRows(actual, rows("0.0"));
+  }
 }