Support mapping _time field access to @timestamp

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

Author: yuancu

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

@@ -502,6 +502,8 @@ public RexNode visitFunction(Function node, CalcitePlanContext context) {
           lambdaNode = analyze(arg, lambdaContext);
         }
         arguments.add(lambdaNode);
+      } else if (arg instanceof Field) {
+        arguments.add(resolveSpecialTimeFieldReference((Field) arg, context));
       } else {
         arguments.add(analyze(arg, context));
       }
@@ -516,6 +518,30 @@ public RexNode visitFunction(Function node, CalcitePlanContext context) {
     throw new IllegalArgumentException("Unsupported operator: " + node.getFuncName());
   }
 
+  /**
+   * Replace {@code _time} field reference to {@code @timestamp} field if {@code _time} does not
+   * exist in the input schema.
+   */
+  private RexNode resolveSpecialTimeFieldReference(Field field, CalcitePlanContext context) {
+    if (field.getField() instanceof QualifiedName && "_time".equals(field.getField().toString())) {
+      try {
+        // Use _time field if it exists
+        return analyze(field, context);
+      } catch (IllegalArgumentException ignored) {
+        try {
+          // Substitute _time field to @timestamp
+          return referenceImplicitTimestampField(context);
+        } catch (IllegalArgumentException e) {
+          throw new SemanticCheckException(
+              "Referring _time field requires an explicit '_time' field or an implicit"
+                  + " '@timestamp' field, but none of them was found in the input schema.",
+              e);
+        }
+      }
+    }
+    return analyze(field, context);
+  }
+
   @Override
   public RexNode visitWindowFunction(WindowFunction node, CalcitePlanContext context) {
     Function windowFunction = (Function) node.getFunction();

core/src/main/java/org/opensearch/sql/calcite/plan/OpenSearchConstants.java

@@ -21,6 +21,7 @@ public interface OpenSearchConstants {
   String METADATA_FIELD_ROUTING = "_routing";
 
   String IMPLICIT_FIELD_TIMESTAMP = "@timestamp";
+  String SPECIAL_FIELD_TIME = "_time";
 
   java.util.Map<String, ExprType> METADATAFIELD_TYPE_MAP =
       Map.of(

core/src/main/java/org/opensearch/sql/executor/QueryService.java

@@ -262,7 +262,12 @@ public RelNode optimize(RelNode plan, CalcitePlanContext context) {
 
   private boolean isCalciteFallbackAllowed() {
     if (settings != null) {
-      return settings.getSettingValue(Settings.Key.CALCITE_FALLBACK_ALLOWED);
+      try {
+        return settings.getSettingValue(Settings.Key.CALCITE_FALLBACK_ALLOWED);
+      } catch (Exception e) {
+        // If failed to retrieve the setting, default fallback to false
+        return false;
+      }
     } else {
       return true;
     }

integ-test/src/test/java/org/opensearch/sql/calcite/standalone/CalcitePPLDateTimeBuiltinFunctionIT.java

@@ -30,8 +30,10 @@
 import org.json.JSONObject;
 import org.junit.jupiter.api.Test;
 import org.opensearch.client.Request;
+import org.opensearch.sql.common.utils.StringUtils;
 import org.opensearch.sql.data.model.ExprDateValue;
 import org.opensearch.sql.data.model.ExprIntegerValue;
+import org.opensearch.sql.exception.SemanticCheckException;
 import org.opensearch.sql.expression.function.FunctionProperties;
 
 public class CalcitePPLDateTimeBuiltinFunctionIT extends CalcitePPLIntegTestCase {
@@ -46,6 +48,7 @@ public void init() throws IOException {
     loadIndex(Index.DATE);
     loadIndex(Index.PEOPLE2);
     loadIndex(Index.BANK);
+    loadIndex(Index.BIG5);
     initRelativeDocs();
   }
 
@@ -1476,6 +1479,25 @@ public void testMicrosecond() throws IOException {
     verifyDataRows(actual, rows(0, 0, 0, 123456, 123456));
   }
 
+  @Test
+  public void testAccessImplicitTimestampField() {
+    JSONObject actual = executeQuery("source=big5 | eval t = unix_timestamp(_time) | fields t");
+    verifySchema(actual, schema("t", "double"));
+    verifyDataRows(actual, rows(1672696954));
+
+    Throwable t =
+        assertThrowsWithReplace(
+            SemanticCheckException.class,
+            () ->
+                executeQuery(
+                    StringUtils.format(
+                        "source=%s | eval t = unix_timestamp(_time)", TEST_INDEX_DATE_FORMATS)));
+    verifyErrorMessageContains(
+        t,
+        "Referring _time field requires an explicit '_time' field or an implicit"
+            + " '@timestamp' field, but none of them was found in the input schema.");
+  }
+
   private static int getYearWeek(LocalDate date, int mode) {
     return exprYearweek(new ExprDateValue(date), new ExprIntegerValue(mode)).integerValue();
   }