Add LogicalPlanSignature and use in the optimizer loop #5623
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mslapek
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I really like this idea, and the code is well commented and tested - thank you @mslapek . 🏅
In general I think the chance of hash collisions is very small.
Even if collisions happen, I think the result will be that a plan is not optimized as much as it might otherwise be (the loop will exit earlier than it might otherwise), which seems like a reasonable outcome to me, especially given the low chance of collisions
| @@ -419,6 +412,34 @@ impl Optimizer { | |||
| } | |||
| } | |||
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| /// Returns an error if plans have different schemas. | |||
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The Boy Scout Rule: "Leave the campground cleaner than you found it" 🤣
| /// When two [`LogicalPlan`]s differ only in metadata, then they will have | ||
| /// the same [`LogicalPlanSignature`]s (due to hash implementation in | ||
| /// [`LogicalPlan`]). | ||
| pub fn compute(plan: &LogicalPlan) -> Self { |
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I think pub fn new() would be more idiomatic to construct a new plan
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@alamb @jackwener Thanks for the review! 🪀 |
| // HashSet::insert returns, whether the value was newly inserted. | ||
| let plan_is_fresh = | ||
| previous_plans.insert(LogicalPlanSignature::new(&new_plan)); | ||
| if !plan_is_fresh { |
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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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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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@mslapek thanks for response!
What motivated you to review this merged (one year ago) PR?
I find the source PR as an efficient way to ask a question about the code and reach people with context -- code author(s) and reviewers -- in one shot.
Yes, a cycle shows a logical error in the optimizer - because at each step the performance should be improved.
Agreed!
At the same time, it does NOT imply a correctness error. The plan probably will be still correct (but not optimal).
Agreed!
We should take the perspective of a user. If a data scientist does research, should we harm the availability of the database?
Suboptimal plan can be orders of magnitude more expensive to execute, so allowing it to run may cause unavailability for others, but I see your point. It's especially difficult to transition from bug lenient treatment to more strict. It should be gradual. I like the idea of having this initially controlled with a flag. In tests and on CI this flag should be set to "fail". Then we can switch the flag on for runtime as well.
@alamb @jackwener thoughts?
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Thank you for bringing this up @mslapek and @findepi
Suboptimal plan can be orders of magnitude more expensive to execute, so allowing it to run may cause unavailability for others, but I see your point. It's especially difficult to transition from bug lenient treatment to more strict. It should be gradual. I like the idea of having this initially controlled with a flag. In tests and on CI this flag should be set to "fail". Then we can switch the flag on for runtime as well.
My thoughts are that I can see the tradeoffs with both behavior:
- Fail fast (raise an error if cycle is detected) that @findepi increases the liklihood that such an error would be found and fixed quickly (as it would prevent the query in question from running)
- Lenient (ignore error) user still gets their answer (though maybe woudl take much longer)
One middle ground might be as @findepi suggests and use a flag -- we could default to raising an error if a cycle was detected but have a way for users to ignore the error and proceed anyways. As long as the error message said how to work around the error I think it would be fine.
In fact we have a similar setting already for failed optimizer rules, for many of the same reasons discussed, that we could model the behavior on datafusion.optimizer.skip_failed_rules: https://datafusion.apache.org/user-guide/configs.html
| datafusion.optimizer.skip_failed_rules | false | When set to true, the logical plan optimizer will produce warning messages if any optimization rules produce errors and then proceed to the next rule. When set to false, any rules that produce errors will cause the query to fail |
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Which issue does this PR close?
Closes #3892
Removes a blocker for #4628 (allows to have only one
LogicalPlanin RAM)Attractive alternative for returning
Option<LogicalPlan>in the optimizer loop (see the comment #4628 (comment))Rationale for this change
Explained in the issue.
What changes are included in this PR?
Added a new structure
LogicalPlanSignaturewithcomputemethod.Used
LogicalPlanSignaturein the main optimization loop to detect cycles with hash-set.(Maybe we should raise default
max_passesin optimizer? We can detect cycles.)Are these changes tested?
Added unit-test for computation in
LogicalPlanSignature(forget_node_numberfunction).Added unit-tests to
optimizer.rstesting cycle-detection ability.Existing tests to detect regression.
Are there any user-facing changes?
NO API changes. The new module with
LogicalPlanSignatureis private (modvspub use).