Implement timestamp_cast_dtype for SqliteDialect (#99)

* Use text for sqlite timestamp

* Add test

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

@@ -3152,4 +3152,30 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn test_cast_timestamp_sqlite() -> Result<()> {
+        let sqlite_dialect: Arc<dyn Dialect> = Arc::new(SqliteDialect {});
+
+        for (dialect, expected) in [
+            (sqlite_dialect, "CAST(`a` AS TEXT)"),
+        ] {
+            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 = expected.to_string();
+
+            assert_eq!(actual, expected);
+        }
+        Ok(())
+    }
 }