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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