Minor: Document ExecutionPlan::equivalence_properties more thorough…

…ly (apache#8128)

* Minor: Document ExecutionPlan::equivalence_properties more thoroughly

* Apply suggestions from code review

Co-authored-by: Liang-Chi Hsieh <[email protected]>
Co-authored-by: Mustafa Akur <[email protected]>

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Co-authored-by: Liang-Chi Hsieh <[email protected]>
Co-authored-by: Mustafa Akur <[email protected]>

datafusion/physical-plan/src/lib.rs

@@ -203,7 +203,23 @@ pub trait ExecutionPlan: Debug + DisplayAs + Send + Sync {
             .collect()
     }
 
-    /// Get the [`EquivalenceProperties`] within the plan
+    /// Get the [`EquivalenceProperties`] within the plan.
+    ///
+    /// Equivalence properties tell DataFusion what columns are known to be
+    /// equal, during various optimization passes. By default, this returns "no
+    /// known equivalences" which is always correct, but may cause DataFusion to
+    /// unnecessarily resort data.
+    ///
+    /// If this ExecutionPlan makes no changes to the schema of the rows flowing
+    /// through it or how columns within each row relate to each other, it
+    /// should return the equivalence properties of its input. For
+    /// example, since `FilterExec` may remove rows from its input, but does not
+    /// otherwise modify them, it preserves its input equivalence properties.
+    /// However, since `ProjectionExec` may calculate derived expressions, it
+    /// needs special handling.
+    ///
+    /// See also [`Self::maintains_input_order`] and [`Self::output_ordering`]
+    /// for related concepts.
     fn equivalence_properties(&self) -> EquivalenceProperties {
         EquivalenceProperties::new(self.schema())
     }