Avoid unnecessary allocations (#559)

* Avoid unnecessary to_string allocations

* Only get displayer for the interesting field

src/array/display.rs

@@ -209,14 +209,9 @@ pub fn get_value_display<'a>(array: &'a dyn Array) -> Box<dyn Fn(usize) -> Strin
         }
         Union(_, _, _) => {
             let array = array.as_any().downcast_ref::<UnionArray>().unwrap();
-            let displays = array
-                .fields()
-                .iter()
-                .map(|x| get_display(x.as_ref()))
-                .collect::<Vec<_>>();
             Box::new(move |row: usize| {
                 let (field, index) = array.index(row);
-                displays[field](index)
+                get_display(array.fields()[field].as_ref())(index)
             })
         }
         Extension(_, _, _) => todo!(),

src/ffi/schema.rs

@@ -380,10 +380,10 @@ fn to_format(data_type: &DataType) -> String {
         }
         DataType::Timestamp(unit, tz) => {
             let unit = match unit {
-                TimeUnit::Second => "s".to_string(),
-                TimeUnit::Millisecond => "m".to_string(),
-                TimeUnit::Microsecond => "u".to_string(),
-                TimeUnit::Nanosecond => "n".to_string(),
+                TimeUnit::Second => "s",
+                TimeUnit::Millisecond => "m",
+                TimeUnit::Microsecond => "u",
+                TimeUnit::Nanosecond => "n",
             };
             format!(
                 "ts{}:{}",