## What changes were proposed in this pull request?
In the Planner, we collect the placeholder which need to be substituted in the query execution plan and once we plan them, we substitute the placeholder with the effective plan.
In this second phase, we rely on the `==` comparison, ie. the `equals` method. This means that if two placeholder plans - which are different instances - have the same attributes (so that they are equal, according to the equal method) they are both substituted with their corresponding new physical plans. So, in such a situation, the first time we substitute both them with the first of the 2 new generated plan and the second time we substitute nothing.
This is usually of no harm for the execution of the query itself, as the 2 plans are identical. But since they are the same instance, now, the local variables are shared (which is unexpected). This causes issues for the metrics collected, as the same node is executed 2 times, so the metrics are accumulated 2 times, wrongly.
The PR proposes to use the `eq` method in checking which placeholder needs to be substituted,; thus in the previous situation, actually both the two different physical nodes which are created (one for each time the logical plan appears in the query plan) are used and the metrics are collected properly for each of them.
## How was this patch tested?
added UT
Closes#22284 from mgaido91/SPARK-25278.
Authored-by: Marco Gaido <marcogaido91@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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