Implement timestamp_cast_dtype for SqliteDialect (apache#17479)

* Implement timestamp_cast_dtype for SqliteDialect

* Fix lint

Author: krinart

datafusion/sql/src/unparser/dialect.rs

@@ -487,6 +487,14 @@ impl Dialect for SqliteDialect {
         false
     }
 
+    fn timestamp_cast_dtype(
+        &self,
+        _time_unit: &TimeUnit,
+        _tz: &Option<Arc<str>>,
+    ) -> ast::DataType {
+        ast::DataType::Text
+    }
+
     fn scalar_function_to_sql_overrides(
         &self,
         unparser: &Unparser,

datafusion/sql/src/unparser/expr.rs

@@ -3173,4 +3173,24 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn test_cast_timestamp_sqlite() -> Result<()> {
+        let dialect: Arc<dyn Dialect> = Arc::new(SqliteDialect {});
+
+        let unparser = Unparser::new(dialect.as_ref());
+        let expr = Expr::Cast(Cast {
+            expr: Box::new(col("a")),
+            data_type: DataType::Timestamp(TimeUnit::Nanosecond, None),
+        });
+
+        let ast = unparser.expr_to_sql(&expr)?;
+
+        let actual = ast.to_string();
+        let expected = "CAST(`a` AS TEXT)".to_string();
+
+        assert_eq!(actual, expected);
+
+        Ok(())
+    }
 }