add more sqlness

Signed-off-by: fys <fengys1996@gmail.com>

src/datatypes/src/vectors/json/array.rs

@@ -205,8 +205,8 @@ impl JsonArray<'_> {
                     if self.inner.is_null(i) {
                         return Ok(None);
                     }
-                    let value = self.try_get_value(i)?;
-                    let value = match value {
+                    let value = match self.try_get_value(i)? {
+                        Value::Null => return Ok(None),
                         Value::String(value) => value,
                         value => value.to_string(),
                     };
@@ -407,6 +407,29 @@ mod test {
         Ok(())
     }
 
+    #[test]
+    fn test_cast_variant_to_utf8_view_preserves_json_null() -> Result<()> {
+        let encode = |json: &[u8]| jsonb::parse_value(json).unwrap().to_vec();
+        let json_null = encode(b"null");
+        let object = encode(br#"{"value":1}"#);
+        let string = encode(br#""text""#);
+        let variants: ArrayRef = Arc::new(BinaryArray::from(vec![
+            Some(json_null.as_slice()),
+            Some(object.as_slice()),
+            Some(string.as_slice()),
+            None,
+        ]));
+
+        let casted = JsonArray::from(&variants).try_cast(&DataType::Utf8View)?;
+        let casted = casted.as_string_view();
+        assert!(casted.is_null(0));
+        assert_eq!(casted.value(1), r#"{"value":1}"#);
+        assert_eq!(casted.value(2), "text");
+        assert!(casted.is_null(3));
+
+        Ok(())
+    }
+
     #[test]
     fn test_align_json_array() -> Result<()> {
         struct TestCase {

tests/cases/standalone/common/types/json/json2.result

@@ -218,15 +218,15 @@ select j.d from json2_table order by ts;
 +-----------------------------------+
 | json_get(json2_table.j,Utf8("d")) |
 +-----------------------------------+
-| [{e: {f: 0.1, g: }}]              |
-| [{e: {f: 0.2, g: }}]              |
+| [{"e":{"f":0.1,"g":null}}]        |
+| [{"e":{"f":0.2,"g":null}}]        |
 |                                   |
-| [{e: {f: , g: -0.4}}]             |
+| [{"e":{"f":null,"g":-0.4}}]       |
 |                                   |
 |                                   |
-| [{e: {g: -0.7}}]                  |
+| [{"e":{"g":-0.7}}]                |
 |                                   |
-| [{e: {g: -0.9}}]                  |
+| [{"e":{"g":-0.9}}]                |
 |                                   |
 +-----------------------------------+
 
@@ -234,6 +234,61 @@ drop table json2_table;
 
 Affected Rows: 0
 
+-- A JSON null in a dynamically typed field must remain SQL NULL when the
+-- field is projected to the default string type, rather than becoming "null".
+create table json2_variant_null (
+    ts timestamp time index,
+    j json2
+) with (
+    'append_mode' = 'true',
+    'sst_format' = 'flat'
+);
+
+Affected Rows: 0
+
+insert into json2_variant_null values
+    (1, '{"payload":{"value":1}}'),
+    (2, '{"payload":"text"}'),
+    (3, '{"payload":null}');
+
+Affected Rows: 3
+
+select ts, j.payload, j.payload is null as payload_is_null
+from json2_variant_null
+order by ts;
+
++-------------------------+------------------------------------------------+-----------------+
+| ts                      | json_get(json2_variant_null.j,Utf8("payload")) | payload_is_null |
++-------------------------+------------------------------------------------+-----------------+
+| 1970-01-01T00:00:00.001 | {"value":1}                                    | false           |
+| 1970-01-01T00:00:00.002 | text                                           | false           |
+| 1970-01-01T00:00:00.003 |                                                | true            |
++-------------------------+------------------------------------------------+-----------------+
+
+admin flush_table('json2_variant_null');
+
++-----------------------------------------+
+| ADMIN flush_table('json2_variant_null') |
++-----------------------------------------+
+| 0                                       |
++-----------------------------------------+
+
+select ts, j.payload, j.payload is null as payload_is_null
+from json2_variant_null
+order by ts;
+
++-------------------------+------------------------------------------------+-----------------+
+| ts                      | json_get(json2_variant_null.j,Utf8("payload")) | payload_is_null |
++-------------------------+------------------------------------------------+-----------------+
+| 1970-01-01T00:00:00.001 | {"value":1}                                    | false           |
+| 1970-01-01T00:00:00.002 | text                                           | false           |
+| 1970-01-01T00:00:00.003 |                                                | true            |
++-------------------------+------------------------------------------------+-----------------+
+
+drop table json2_variant_null;
+
+Affected Rows: 0
+
 create table json2_without_append_mode (
     ts timestamp time index,
     j json2

tests/cases/standalone/common/types/json/json2.sql

@@ -73,6 +73,33 @@ select j.d from json2_table order by ts;
 
 drop table json2_table;
 
+-- A JSON null in a dynamically typed field must remain SQL NULL when the
+-- field is projected to the default string type, rather than becoming "null".
+create table json2_variant_null (
+    ts timestamp time index,
+    j json2
+) with (
+    'append_mode' = 'true',
+    'sst_format' = 'flat'
+);
+
+insert into json2_variant_null values
+    (1, '{"payload":{"value":1}}'),
+    (2, '{"payload":"text"}'),
+    (3, '{"payload":null}');
+
+select ts, j.payload, j.payload is null as payload_is_null
+from json2_variant_null
+order by ts;
+
+admin flush_table('json2_variant_null');
+
+select ts, j.payload, j.payload is null as payload_is_null
+from json2_variant_null
+order by ts;
+
+drop table json2_variant_null;
+
 create table json2_without_append_mode (
     ts timestamp time index,
     j json2