Align DataFrame::fill_null column argument with fill_nan

datafusion/core/src/dataframe/mod.rs

@@ -2439,7 +2439,7 @@ impl DataFrame {
     }
 
     /// Fill null values in specified columns with a given value
-    /// If no columns are specified (empty vector), applies to all columns
+    /// If no columns are specified (empty slice), applies to all columns
     /// Only fills if the value can be cast to the column's type
     ///
     /// # Arguments
@@ -2458,19 +2458,14 @@ impl DataFrame {
     ///     .read_csv("tests/data/example.csv", CsvReadOptions::new())
     ///     .await?;
     /// // Fill nulls in only columns "a" and "c":
-    /// let df = df.fill_null(ScalarValue::from(0), vec!["a".to_owned(), "c".to_owned()])?;
+    /// let df = df.fill_null(&ScalarValue::from(0), &["a", "c"])?;
     /// // Fill nulls across all columns:
-    /// let df = df.fill_null(ScalarValue::from(0), vec![])?;
+    /// let df = df.fill_null(&ScalarValue::from(0), &[])?;
     /// # Ok(())
     /// # }
     /// ```
-    #[expect(clippy::needless_pass_by_value)]
-    pub fn fill_null(
-        &self,
-        value: ScalarValue,
-        columns: Vec<String>,
-    ) -> Result<DataFrame> {
-        self.fill_columns(&value, &columns, &coalesce(), |_| true)
+    pub fn fill_null(&self, value: &ScalarValue, columns: &[&str]) -> Result<DataFrame> {
+        self.fill_columns(value, columns, &coalesce(), |_| true)
     }
 
     // Helper to find columns from names

datafusion/core/tests/dataframe/mod.rs

@@ -6472,11 +6472,8 @@ async fn test_fill_null() -> Result<()> {
 
     // Use fill_null to replace nulls on each column.
     let df_filled = df
-        .fill_null(ScalarValue::Int32(Some(0)), vec!["a".to_string()])?
-        .fill_null(
-            ScalarValue::Utf8(Some("default".to_string())),
-            vec!["b".to_string()],
-        )?;
+        .fill_null(&ScalarValue::Int32(Some(0)), &["a"])?
+        .fill_null(&ScalarValue::Utf8(Some("default".to_string())), &["b"])?;
 
     let results = df_filled.collect().await?;
     assert_snapshot!(
@@ -6502,8 +6499,7 @@ async fn test_fill_null_all_columns() -> Result<()> {
     // Use fill_null to replace nulls on all columns.
     // Only column "b" will be replaced since ScalarValue::Utf8(Some("default".to_string()))
     // can be cast to Utf8.
-    let df_filled =
-        df.fill_null(ScalarValue::Utf8(Some("default".to_string())), vec![])?;
+    let df_filled = df.fill_null(&ScalarValue::Utf8(Some("default".to_string())), &[])?;
 
     let results = df_filled.clone().collect().await?;
 
@@ -6521,7 +6517,7 @@ async fn test_fill_null_all_columns() -> Result<()> {
     );
 
     // Fill column "a" null values with a value that cannot be cast to Int32.
-    let df_filled = df_filled.fill_null(ScalarValue::Int32(Some(0)), vec![])?;
+    let df_filled = df_filled.fill_null(&ScalarValue::Int32(Some(0)), &[])?;
 
     let results = df_filled.collect().await?;
     assert_snapshot!(

docs/source/library-user-guide/upgrading/55.0.0.md

@@ -25,6 +25,48 @@
 in this section pertains to features and changes that have already been merged
 to the main branch and are awaiting release in this version.
 
+### `DataFrame::fill_null` now borrows its arguments
+
+`DataFrame::fill_null` previously took its arguments by value:
+
+```rust,ignore
+// Before
+pub fn fill_null(
+    &self,
+    value: ScalarValue,
+    columns: Vec<String>,
+) -> Result<DataFrame>
+```
+
+It now borrows them, matching the signature of the newly added
+`DataFrame::fill_nan`:
+
+```rust,ignore
+// After
+pub fn fill_null(
+    &self,
+    value: &ScalarValue,
+    columns: &[&str],
+) -> Result<DataFrame>
+```
+
+This lets callers pass a borrowed `ScalarValue` and slice literals (or
+`&str` column names) without first allocating owned `String`s.
+
+**Migration guide:**
+
+Borrow the value and pass a slice of `&str` instead of an owned `Vec<String>`:
+
+```rust,ignore
+// Before
+let df = df.fill_null(ScalarValue::from(0), vec!["a".to_owned(), "c".to_owned()])?;
+let df = df.fill_null(ScalarValue::from(0), vec![])?;
+
+// After
+let df = df.fill_null(&ScalarValue::from(0), &["a", "c"])?;
+let df = df.fill_null(&ScalarValue::from(0), &[])?;
+```
+
 ### `Dialect::AVAILABLE` replaced by `Dialect::available()`
 
 `datafusion_common::config::Dialect::AVAILABLE` has been removed. Use