more tests and validation of data type conversions

Author: fang-xing-esql

x-pack/plugin/esql/qa/testFixtures/src/main/resources/union_types.csv-spec

@@ -2144,9 +2144,9 @@ required_capability: implicit_casting_date_and_date_nanos
 
 FROM date_nanos, date_nanos_union_types
 | EVAL a = nanos::date_nanos::datetime
+| WHERE millis < "2023-10-23T13:00:00"
 | MV_EXPAND a
 | SORT a, millis, nanos
-| LIMIT 14
 ;
 
 warningRegex:evaluation of \[FROM .*date_nanos.*date_nanos_union_types.*\] failed, treating result as null. Only first 20 failures recorded.
@@ -2167,6 +2167,11 @@ millis:date_nanos              | nanos:date_nanos
 1999-10-23T12:15:03.360103847Z | [2023-01-23T13:55:01.543Z, 2023-02-23T13:33:34.937Z, 2023-03-23T12:15:03.360Z]                   | 0                   | 2023-03-23T12:15:03.360Z
 2023-10-23T12:15:03.360Z       | 2023-10-23T12:15:03.360103847Z                                                                   | 1698063303360103847 | 2023-10-23T12:15:03.360Z
 2023-10-23T12:15:03.360Z       | 2023-10-23T12:15:03.360103847Z                                                                   | 1698063303360103847 | 2023-10-23T12:15:03.360Z
+2023-10-23T12:15:03.360103847Z | 2023-10-23T12:15:03.360Z                                                                         | 1698063303360103847 | 2023-10-23T12:15:03.360Z
+2023-10-23T12:15:03.360103847Z | 2023-10-23T12:15:03.360Z                                                                         | 1698063303360103847 | 2023-10-23T12:15:03.360Z
+2023-10-23T12:27:28.948Z       | 2023-10-23T12:27:28.948Z                                                                         | 1698064048948000000 | 2023-10-23T12:27:28.948Z
+2023-10-23T12:27:28.948Z       | 2023-10-23T12:27:28.948Z                                                                         | 1698064048948000000 | 2023-10-23T12:27:28.948Z
+2023-10-23T12:15:03.360103847Z | null                                                                                             | 1698063303360103847 | null
 ;
 
 ImplicitCastingMultiTypedMVFieldsEval

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/analysis/Analyzer.java

@@ -1697,15 +1697,18 @@ private Expression resolveConvertFunction(ConvertFunction convert, List<FieldAtt
                 && fa.field() instanceof MultiTypeEsField mtf) {
                     // This is an explicit casting of a union typed field that has been converted to MultiTypeEsField in EsRelation by
                     // DateMillisToNanosInEsRelation, it is not necessary to cast it again to the same type, replace the implicit casting
-                    // with explicit casting.
+                    // with explicit casting. However, it is useful to differentiate implicit and explicit casting in some cases, for
+                    // example, an expression like multiTypeEsField(synthetic=false, date_nanos)::date_nanos::datetime is rewritten to
+                    // multiTypeEsField(synthetic=true, date_nanos)::datetime, the implicit casting is overwritten by explicit casting and
+                    // the multiTypeEsField is not casted to datetime directly.
                     if (((Expression) convert).dataType() == mtf.getDataType()) {
                         return createIfDoesNotAlreadyExist(fa, mtf, unionFieldAttributes);
                     }
 
                     // Data type is different between implicit(date_nanos) and explicit casting, if the conversion is supported, create a
                     // new MultiTypeEsField with explicit casting type, and add it to unionFieldAttributes.
                     Set<DataType> supportedTypes = convert.supportedTypes();
-                    if (supportedTypes.contains(fa.dataType())) {
+                    if (supportedTypes.contains(fa.dataType()) && canConvertOriginalTypes(mtf, supportedTypes)) {
                         // Build the mapping between index name and conversion expressions
                         Map<String, Expression> indexToConversionExpressions = new HashMap<>();
                         for (Map.Entry<String, Expression> entry : mtf.getIndexToConversionExpressions().entrySet()) {
@@ -1716,7 +1719,7 @@ private Expression resolveConvertFunction(ConvertFunction convert, List<FieldAtt
                             indexToConversionExpressions.put(indexName, newConvertFunction);
                         }
                         MultiTypeEsField multiTypeEsField = new MultiTypeEsField(
-                            fa.name(),
+                            fa.fieldName(),
                             convertExpression.dataType(),
                             false,
                             indexToConversionExpressions
@@ -1767,6 +1770,16 @@ private static MultiTypeEsField resolvedMultiTypeEsField(
             return MultiTypeEsField.resolveFrom(imf, typesToConversionExpressions);
         }
 
+        private static boolean canConvertOriginalTypes(MultiTypeEsField multiTypeEsField, Set<DataType> supportedTypes) {
+            return multiTypeEsField.getIndexToConversionExpressions()
+                .values()
+                .stream()
+                .allMatch(
+                    e -> e instanceof AbstractConvertFunction convertFunction
+                        && supportedTypes.contains(convertFunction.field().dataType().widenSmallNumeric())
+                );
+        }
+
         private static Expression typeSpecificConvert(ConvertFunction convert, Source source, DataType type, InvalidMappedField mtf) {
             EsField field = new EsField(mtf.getName(), type, mtf.getProperties(), mtf.isAggregatable());
             return typeSpecificConvert(convert, source, field);

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/planner/QueryTranslatorTests.java

@@ -436,5 +436,16 @@ public void testToDateNanos() {
             "2020-01-01T00:00:00.000Z","boost":0.0.*""" + """
             esql_single_value":\\{"field":"date_and_date_nanos".*"must_not".*"term":\\{"date_and_date_nanos":\\{"value":\
             "2025-01-01T00:00:00.000Z","boost":0.0.*"""));
+
+        // explicit casting
+        assertQueryTranslationDateDateNanosUnionTypes(
+            """
+                FROM test* | WHERE date_and_date_nanos::datetime < "2025-12-31" and date_and_date_nanos > "2025-01-01\"""",
+            stats,
+            containsString("""
+                "esql_single_value":{"field":"date_and_date_nanos",\
+                "next":{"range":{"date_and_date_nanos":{"gt":"2025-01-01T00:00:00.000Z","time_zone":"Z",\
+                "format":"strict_date_optional_time_nanos","boost":0.0}}}""")
+        );
     }
 }