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Posted to issues@spark.apache.org by "Xiaoju Wu (Jira)" <ji...@apache.org> on 2019/12/09 12:50:00 UTC
[jira] [Commented] (SPARK-30072) Create dedicated planner for
subqueries
[ https://issues.apache.org/jira/browse/SPARK-30072?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16991554#comment-16991554 ]
Xiaoju Wu commented on SPARK-30072:
-----------------------------------
[~afroozeh] I think the change from checking if queryExecution equals to checking isSubquery boolean is not correct in some scenarios. If a subquery is executed under SubqueryExec.executionContext, there's no way to pass the information to tell InsertAdaptiveSparkPlan rule that it is a subquery.
> Create dedicated planner for subqueries
> ---------------------------------------
>
> Key: SPARK-30072
> URL: https://issues.apache.org/jira/browse/SPARK-30072
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 3.0.0
> Reporter: Ali Afroozeh
> Assignee: Ali Afroozeh
> Priority: Minor
> Fix For: 3.0.0
>
>
> This PR changes subquery planning by calling the planner and plan preparation rules on the subquery plan directly. Before we were creating a QueryExecution instance for subqueries to get the executedPlan. This would re-run analysis and optimization on the subqueries plan. Running the analysis again on an optimized query plan can have unwanted consequences, as some rules, for example DecimalPrecision, are not idempotent.
> As an example, consider the expression 1.7 * avg(a) which after applying the DecimalPrecision rule becomes:
> promote_precision(1.7) * promote_precision(avg(a))
> After the optimization, more specifically the constant folding rule, this expression becomes:
> 1.7 * promote_precision(avg(a))
> Now if we run the analyzer on this optimized query again, we will get:
> promote_precision(1.7) * promote_precision(promote_precision(avg(a)))
> Which will later optimized as:
> 1.7 * promote_precision(promote_precision(avg(a)))
> As can be seen, re-running the analysis and optimization on this expression results in an expression with extra nested promote_preceision nodes. Adding unneeded nodes to the plan is problematic because it can eliminate situations where we can reuse the plan.
> We opted to introduce dedicated planners for subuqueries, instead of making the DecimalPrecision rule idempotent, because this eliminates this entire category of problems. Another benefit is that planning time for subqueries is reduced.
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