[GitHub] [spark] francis0407 opened a new pull request #24344: [SPARK-27440][SQL] Optimize uncorrelated predicate subquery

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francis0407 opened a new pull request #24344: [SPARK-27440][SQL] Optimize uncorrelated predicate subquery
URL: https://github.com/apache/spark/pull/24344
 
 
   ## What changes were proposed in this pull request?
   
   This PR is trying to optimize uncorrelated predicate subqueries(InSubquery, Exists).
   Currently, we rewrite all the predicate subqueries(`InSubquery`, `Exists`) as semi-join/anti-join. But uncorrelated predicate subquery can be evaluated using a subplan instead of a join. We can firstly rewrite all the uncorrelated predicate subqueries as `Exists`, then optimize it and compute it using a subquery physical plan like ScalarSubquery. 
   
   This PR adds a new Optimize rule: RewriteUncorrelatedSubquery.
   This rule rewrites uncorrelated PredicateSubquery expressions as Exists(it can also be used for ANY/SOME/ALL). Besides, we can use `limit 1` and `select 1` after the subquery to reduce the result set. `InSubquery` can be rewritten as uncorrelated Exists only when the left side values are literals and the subquery has no outer reference. Here is an example,
   ```SQL
   3 in (select b from t) => exists(select 1 from t where b = 3 limit 1)
   ```
   
   Also, this PR adds a new class `Exists` which is the physical copy of Exists to be used inside SparkPlan.
   
   
   ## How was this patch tested?
   
   ut
   

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