[GitHub] [arrow-datafusion] tustvold commented on a diff in pull request #5071: DataFrame count method

["tustvold (via GitHub)" <gi...@apache.org>]

tustvold commented on code in PR #5071:
URL: https://github.com/apache/arrow-datafusion/pull/5071#discussion_r1137405371


##########
datafusion/core/src/dataframe.rs:
##########
@@ -361,6 +363,39 @@ impl DataFrame {
         Ok(DataFrame::new(self.session_state, plan))
     }
 
+    /// Run a count aggregate on the DataFrame and execute the DataFrame to collect this
+    /// count and return it as a usize, to find the total number of rows after executing
+    /// the DataFrame.
+    /// ```
+    /// # use datafusion::prelude::*;
+    /// # use datafusion::error::Result;
+    /// # #[tokio::main]
+    /// # async fn main() -> Result<()> {
+    /// let ctx = SessionContext::new();
+    /// let df = ctx.read_csv("tests/data/example.csv", CsvReadOptions::new()).await?;
+    /// let count = df.count().await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn count(self) -> Result<usize> {
+        let rows = self
+            .aggregate(
+                vec![],
+                vec![datafusion_expr::count(Expr::Literal(ScalarValue::Null))],

Review Comment:
   This was actually working by accident as a result of a quirk of NullArray whereby it doesn't have a null buffer despite all values being null. The count reported by this query should be 0 as only non-null values are counted.
   
   Fix in https://github.com/apache/arrow-datafusion/pull/5612/files#diff-932cfd7271917561280a69edabb35cfd109d22ae736f77e85adcf63455918121R630



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