mslapek commented on code in PR #5623:
URL: https://github.com/apache/datafusion/pull/5623#discussion_r1664441817
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datafusion/optimizer/src/optimizer.rs:
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@@ -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:
Yes, a cycle shows a logical error in the optimizer - because at each step
the performance should be improved.
At the same time, it does NOT imply a correctness error. The plan probably
will be still correct (but not optimal).
> IMO cycle should be error condition
We should take the perspective of a user. If a data scientist does research,
should we harm the availability of the database?
IMO Maybe a log of this situation could be useful, with a configuration flag
turning this into an error...
@findepi Btw. What motivated you to review this merged (one year ago) PR?
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