Github user mgaido91 commented on the issue:
https://github.com/apache/spark/pull/22518
@gengliangwang no, let me cite and explain the PR description. I am not
sure how to improve it, but if you have suggestions I am happy to. The main
point of the PR is to address an issue which arise when:
> When a `ExecSubqueryExpression` is copied
Now the point is, can this condition happen? The answer is yes, and one
situation in which this happens (as reported in the JIRA) is
> when a filter containing a scalar subquery is pushed to a DataSource.
So in the plan we have two `ExecSubqueryExpression` each with a copy of the
same `SubqueryExec`. The problem which arises in this condition is that:
> `ReuseSubquery` becomes useless, as replacing the `SubqueryExec` is
ignored since the new plan is equal to the previous one.
So this result in the subquery being executed twice (as the two
`SubqueryExec` are distinct, despite they are the same).
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