fix: Function expr names without arguments are plain function names

Signed-off-by: Alex Qyoun-ae <[email protected]>

Author: MazterQyou

datafusion/core/src/optimizer/simplify_expressions.rs

@@ -1860,7 +1860,7 @@ mod tests {
         // expect the same timestamp appears in both exprs
         let actual = get_optimized_plan_formatted(&plan, &time);
         let expected = format!(
-            "Projection: TimestampNanosecond({}, Some(\"UTC\")) AS now(), TimestampNanosecond({}, Some(\"UTC\")) AS t2\
+            "Projection: TimestampNanosecond({}, Some(\"UTC\")) AS now, TimestampNanosecond({}, Some(\"UTC\")) AS t2\
             \n  TableScan: test projection=None",
             time.timestamp_nanos(),
             time.timestamp_nanos()
@@ -1910,7 +1910,7 @@ mod tests {
 
         // Note that constant folder runs and folds the entire
         // expression down to a single constant (true)
-        let expected = "Filter: Boolean(true) AS CAST(now() AS Int64) < CAST(totimestamp(Utf8(\"2020-09-08T12:05:00+00:00\")) AS Int64) + Int32(50000)\
+        let expected = "Filter: Boolean(true) AS CAST(now AS Int64) < CAST(totimestamp(Utf8(\"2020-09-08T12:05:00+00:00\")) AS Int64) + Int32(50000)\
                         \n  TableScan: test projection=None";
         let actual = get_optimized_plan_formatted(&plan, &time);
 

datafusion/core/tests/sql/functions.rs

@@ -602,7 +602,7 @@ async fn pi_function() -> Result<()> {
     let actual = execute_to_batches(&ctx, sql).await;
     let expected = vec![
         "+-------------------+--------------------+--------------------+",
-        "| pi()              | pi() / Int64(2)    | pi() / Int64(3)    |",
+        "| pi                | pi / Int64(2)      | pi / Int64(3)      |",
         "+-------------------+--------------------+--------------------+",
         "| 3.141592653589793 | 1.5707963267948966 | 1.0471975511965976 |",
         "+-------------------+--------------------+--------------------+",

datafusion/expr/src/expr.rs

@@ -772,6 +772,9 @@ fn create_function_name(
     args: &[Expr],
     input_schema: &DFSchema,
 ) -> Result<String> {
+    if !distinct && args.is_empty() {
+        return Ok(fun.to_ascii_lowercase());
+    }
     let names: Vec<String> = args
         .iter()
         .map(|e| create_name(e, input_schema))