Revert NDV propagation through projections

Remove the single-column expression heuristic for preserving NDV
in projections, as it is too broad to be correct. Expression-level
statistics propagation will be addressed in a follow-up design.

Author: asolimando

datafusion/physical-expr/src/projection.rs

@@ -711,25 +711,9 @@ impl ProjectionExprs {
                     }
                 }
             } else {
-                // TODO: expressions should compute their own statistics
-                //
-                // For now, try to preserve NDV if the expression references a
-                // single column (as a conservative upper bound).
-                // More accurate NDV propagation would require tracking injectivity
-                // of functions (e.g., `a + 1` preserves NDV exactly, `ABS(a)` may
-                // reduce it, `a % 10` bounds it to 10)
-                let columns = collect_columns(expr);
-                if columns.len() == 1 {
-                    let col_idx = columns.iter().next().unwrap().index();
-                    ColumnStatistics {
-                        distinct_count: stats.column_statistics[col_idx]
-                            .distinct_count
-                            .to_inexact(),
-                        ..ColumnStatistics::new_unknown()
-                    }
-                } else {
-                    ColumnStatistics::new_unknown()
-                }
+                // TODO stats: estimate more statistics from expressions
+                // (expressions should compute their statistics themselves)
+                ColumnStatistics::new_unknown()
             };
             column_statistics.push(col_stats);
         }
@@ -2734,11 +2718,10 @@ pub(crate) mod tests {
         // Should have 2 column statistics
         assert_eq!(output_stats.column_statistics.len(), 2);
 
-        // First column (expression `col0 + 1`) preserves NDV from the single
-        // referenced column as a conservative upper bound (marked Inexact)
+        // First column (expression) should have unknown statistics
         assert_eq!(
             output_stats.column_statistics[0].distinct_count,
-            Precision::Inexact(5)
+            Precision::Absent
         );
         assert_eq!(
             output_stats.column_statistics[0].max_value,