add a describe method on DataFrame like Polars

Author: jiangzhx

datafusion/core/src/dataframe.rs

@@ -307,12 +307,16 @@ impl DataFrame {
             let df = self.clone();
             let mut aggr_expr: Vec<Expr> = vec![];
             aggr_expr.push(datafusion_expr::count(col(field.name())));
+            let mut rs = vec![];
             match field.data_type().is_numeric() {
                 true => {
                     aggr_expr.push(datafusion_expr::min(col(field.name())));
                     aggr_expr.push(datafusion_expr::max(col(field.name())));
                 }
-                false => {}
+                false => {
+                    rs.push("null".to_string());
+                    rs.push("null".to_string());
+                }
             }
 
             let df = df
@@ -322,15 +326,7 @@ impl DataFrame {
                 .await
                 .unwrap();
 
-            let mut rs = vec![];
             for batch in df {
-                match batch.num_columns() {
-                    1 => {
-                        rs.push("null".to_string());
-                        rs.push("null".to_string());
-                    }
-                    _ => {}
-                }
                 for col in 0..batch.num_columns() {
                     let column = batch.column(col);
                     let val = array_value_to_string(column, 0).unwrap();