feat: make DataFrame::create_physical_plan take &self instead of self (#20562)

## Which issue does this PR close?

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- Closes #20561

## Rationale for this change

Previously `create_physical_plan` consumed the `DataFrame`, making it
impossible to inspect (e.g. log) the physical plan and then execute the
same `DataFrame` (e.g. via `write_parquet` or `collect`) without first
cloning it.

Since the method only needs `&LogicalPlan` (which it forwards to
`SessionState::create_physical_plan`), there is no reason to take
ownership. Changing the signature to `&self` makes the common pattern of
"get plan for logging, then write/collect" work naturally.


Also removes the now-unnecessary `self.clone()` in `DataFrame::cache`
that was introduced for the same reason.


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## What changes are included in this PR?
Changing `self` to `&self`
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## Are these changes tested?
Yes
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---------

Co-authored-by: xanderbailey <xanderbailey@users.noreply.github.com>

Author: xanderbailey

datafusion/core/src/dataframe/mod.rs

@@ -295,8 +295,11 @@ impl DataFrame {
         self.session_state.create_logical_expr(sql, df_schema)
     }
 
-    /// Consume the DataFrame and produce a physical plan
-    pub async fn create_physical_plan(self) -> Result<Arc<dyn ExecutionPlan>> {
+    /// Create a physical plan from this DataFrame.
+    ///
+    /// The `DataFrame` remains accessible after this call, so you can inspect
+    /// the plan and still call [`DataFrame::collect`] or other execution methods.
+    pub async fn create_physical_plan(&self) -> Result<Arc<dyn ExecutionPlan>> {
         self.session_state.create_physical_plan(&self.plan).await
     }
 
@@ -2398,7 +2401,7 @@ impl DataFrame {
         } else {
             let context = SessionContext::new_with_state((*self.session_state).clone());
             // The schema is consistent with the output
-            let plan = self.clone().create_physical_plan().await?;
+            let plan = self.create_physical_plan().await?;
             let schema = plan.schema();
             let task_ctx = Arc::new(self.task_ctx());
             let partitions = collect_partitioned(plan, task_ctx).await?;

datafusion/core/src/dataframe/parquet.rs

@@ -610,4 +610,25 @@ mod tests {
 
         Ok(())
     }
+
+    /// Test that `create_physical_plan` does not consume the `DataFrame`, so
+    /// callers can inspect (e.g. log) the physical plan and then still call
+    /// `write_parquet` or any other execution method on the same `DataFrame`.
+    #[tokio::test]
+    async fn create_physical_plan_does_not_consume_dataframe() -> Result<()> {
+        use crate::prelude::CsvReadOptions;
+        let ctx = SessionContext::new();
+        let df = ctx
+            .read_csv("tests/data/example.csv", CsvReadOptions::new())
+            .await?;
+
+        // Obtain the physical plan for inspection without consuming `df`.
+        let _physical_plan = df.create_physical_plan().await?;
+
+        // `df` is still usable — collect the results.
+        let batches = df.collect().await?;
+        assert!(!batches.is_empty());
+
+        Ok(())
+    }
 }