Fix casting double 0.0 to string

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

Author: yuancu

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

@@ -15,6 +15,7 @@
 import org.apache.calcite.rex.RexLiteral;
 import org.apache.calcite.rex.RexNode;
 import org.apache.calcite.sql.SqlIntervalQualifier;
+import org.apache.calcite.sql.fun.SqlLibraryOperators;
 import org.apache.calcite.sql.fun.SqlStdOperatorTable;
 import org.apache.calcite.sql.parser.SqlParserPos;
 import org.apache.calcite.sql.type.SqlTypeName;
@@ -28,9 +29,17 @@
 import org.opensearch.sql.expression.function.PPLBuiltinOperators;
 
 public class ExtendedRexBuilder extends RexBuilder {
+  /**
+   * Formats double values in PPL using a non-scientific notation, displaying up to 16 digits after
+   * the decimal point. This formatting is consistent with PPL V2.
+   */
+  private static final String DOUBLE_FORMAT = "0.0###############";
+
+  private final RexLiteral doubleFormat;
 
   public ExtendedRexBuilder(RexBuilder rexBuilder) {
     super(rexBuilder.getTypeFactory());
+    doubleFormat = makeLiteral(DOUBLE_FORMAT);
   }
 
   public RexNode coalesce(RexNode... nodes) {
@@ -150,6 +159,15 @@ public RexNode makeCast(
             String.format(Locale.ROOT, "Cannot cast from %s to %s", argExprType, udt.name()));
       };
     }
+    // If casting an approximate numeric (e.g. double) to a character type and no format is
+    // specified,
+    // use the custom double format to ensure non\-scientific notation with up to 16 decimal digits.
+    // 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)
+        && format.getType().getSqlTypeName() == SqlTypeName.NULL) {
+      return makeCall(type, SqlLibraryOperators.FORMAT_NUMBER, List.of(exp, doubleFormat));
+    }
     return super.makeCast(pos, type, exp, matchNullability, safe, format);
   }
 }

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

@@ -5,6 +5,7 @@
 
 package org.opensearch.sql.ppl;
 
+import static org.opensearch.sql.legacy.TestsConstants.TEST_INDEX_ACCOUNT;
 import static org.opensearch.sql.legacy.TestsConstants.TEST_INDEX_DATATYPE_NONNUMERIC;
 import static org.opensearch.sql.legacy.TestsConstants.TEST_INDEX_DATATYPE_NUMERIC;
 import static org.opensearch.sql.legacy.TestsConstants.TEST_INDEX_DATE_FORMATS;
@@ -18,6 +19,7 @@
 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 +434,17 @@ 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_ACCOUNT));
+    verifySchema(actual, schema("s", "string"));
+    verifyDataRows(actual, rows("0.0"));
+  }
 }