fix: prevent duplicate alias collision with user-provided __datafusion_extracted names

datafusion/common/src/alias.rs

@@ -37,6 +37,16 @@ impl AliasGenerator {
         Self::default()
     }
 
+    /// Advance the counter to at least `min_id`, ensuring future aliases
+    /// won't collide with already-existing ones.
+    ///
+    /// For example, if the query already contains an alias `alias_42`, then calling
+    /// `update_min_id(42)` will ensure that future aliases generated by this
+    /// [`AliasGenerator`] will start from `alias_43`.
+    pub fn update_min_id(&self, min_id: usize) {
+        self.next_id.fetch_max(min_id + 1, Ordering::Relaxed);
+    }
+
     /// Return a unique alias with the provided prefix
     pub fn next(&self, prefix: &str) -> String {
         let id = self.next_id.fetch_add(1, Ordering::Relaxed);

datafusion/optimizer/src/extract_leaf_expressions.rs

@@ -114,10 +114,6 @@ impl OptimizerRule for ExtractLeafExpressions {
         "extract_leaf_expressions"
     }
 
-    fn apply_order(&self) -> Option<ApplyOrder> {
-        Some(ApplyOrder::TopDown)
-    }
-
     fn rewrite(
         &self,
         plan: LogicalPlan,
@@ -127,10 +123,45 @@ impl OptimizerRule for ExtractLeafExpressions {
             return Ok(Transformed::no(plan));
         }
         let alias_generator = config.alias_generator();
-        extract_from_plan(plan, alias_generator)
+
+        // Advance the alias generator past any user-provided __datafusion_extracted_N
+        // aliases to prevent collisions when generating new extraction aliases.
+        advance_generator_past_existing(&plan, alias_generator)?;
+
+        plan.transform_down_with_subqueries(|plan| {
+            extract_from_plan(plan, alias_generator)
+        })
     }
 }
 
+/// Scans the current plan node's expressions for pre-existing
+/// `__datafusion_extracted_N` aliases and advances the generator
+/// counter past them to avoid collisions with user-provided aliases.
+fn advance_generator_past_existing(
+    plan: &LogicalPlan,
+    alias_generator: &AliasGenerator,
+) -> Result<()> {
+    plan.apply(|plan| {
+        plan.expressions().iter().try_for_each(|expr| {
+            expr.apply(|e| {
+                if let Expr::Alias(alias) = e
+                    && let Some(id_str) = alias
+                        .name
+                        .strip_prefix(EXTRACTED_EXPR_PREFIX)
+                        .and_then(|s| s.strip_prefix('_'))
+                    && let Ok(id) = id_str.parse::<usize>()
+                {
+                    alias_generator.update_min_id(id);
+                }
+                Ok(TreeNodeRecursion::Continue)
+            })?;
+            Ok::<(), datafusion_common::error::DataFusionError>(())
+        })?;
+        Ok(TreeNodeRecursion::Continue)
+    })
+    .map(|_| ())
+}
+
 /// Extracts `MoveTowardsLeafNodes` sub-expressions from a plan node.
 ///
 /// Works for any number of inputs (0, 1, 2, …N). For multi-input nodes

datafusion/sqllogictest/test_files/projection_pushdown.slt

@@ -1949,3 +1949,27 @@ ORDER BY simple_struct.id;
 3 3
 4 4
 5 5
+
+# =========================================================================
+# Regression: user-provided __datafusion_extracted aliases must not
+# collide with optimizer-generated ones
+# (https://github.com/apache/datafusion/issues/20430)
+# =========================================================================
+
+statement ok
+COPY ( select {f1: 1, f2: 2} as s
+) TO 'test_files/scratch/projection_pushdown/test.parquet'
+STORED AS PARQUET;
+
+statement ok
+CREATE EXTERNAL TABLE t
+STORED AS PARQUET
+LOCATION 'test_files/scratch/projection_pushdown/test.parquet';
+
+query I
+SELECT
+    get_field(s, 'f1') AS __datafusion_extracted_2
+FROM t
+WHERE COALESCE(get_field(s, 'f1'), get_field(s, 'f2')) = 1;
+----
+1