Re: [PR] Add LogicalPlanSignature and use in the optimizer loop [datafusion]

["findepi (via GitHub)" <gi...@apache.org>]

findepi commented on code in PR #5623:
URL: https://github.com/apache/datafusion/pull/5623#discussion_r1664186593


##########
datafusion/optimizer/src/optimizer.rs:
##########
@@ -330,15 +324,14 @@ impl Optimizer {
             }
             log_plan(&format!("Optimized plan (pass {i})"), &new_plan);
 
-            // TODO this is an expensive way to see if the optimizer did anything and
-            // it would be better to change the OptimizerRule trait to return an Option
-            // instead
-            if old_plan.as_ref() == &new_plan {
+            // HashSet::insert returns, whether the value was newly inserted.
+            let plan_is_fresh =
+                previous_plans.insert(LogicalPlanSignature::new(&new_plan));
+            if !plan_is_fresh {

Review Comment:
   At this point we detected optimization cycle. Cycles are bad, so we exit. Exiting isn't ideal because our plan is optimized yet. We simply stopped applying rules.
   
   IMO cycle should be error condition



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