### What changes were proposed in this pull request?
This PR combines the current plan and the initial plan in the AQE query plan string when the two plans are the same. It also removes the `== Current Plan ==` and `== Initial Plan ==` headers:
Before
```scala
AdaptiveSparkPlan isFinalPlan=false
+- == Current Plan ==
SortMergeJoin [key#13], [a#23], Inner
:- Sort [key#13 ASC NULLS FIRST], false, 0
: +- Exchange hashpartitioning(key#13, 5), true, [id=#94]
...
+- == Initial Plan ==
SortMergeJoin [key#13], [a#23], Inner
:- Sort [key#13 ASC NULLS FIRST], false, 0
: +- Exchange hashpartitioning(key#13, 5), true, [id=#94]
...
```
After
```scala
AdaptiveSparkPlan isFinalPlan=false
+- SortMergeJoin [key#13], [a#23], Inner
:- Sort [key#13 ASC NULLS FIRST], false, 0
: +- Exchange hashpartitioning(key#13, 5), true, [id=#94]
...
```
For SQL `EXPLAIN` output:
Before
```scala
AdaptiveSparkPlan (8)
+- == Current Plan ==
Sort (7)
+- Exchange (6)
...
+- == Initial Plan ==
Sort (7)
+- Exchange (6)
...
```
After
```scala
AdaptiveSparkPlan (8)
+- Sort (7)
+- Exchange (6)
...
```
### Why are the changes needed?
To simplify the AQE plan string by removing the redundant plan information.
### Does this PR introduce _any_ user-facing change?
Yes.
### How was this patch tested?
Modified the existing unit test.
Closes#29915 from allisonwang-db/aqe-explain.
Authored-by: allisonwang-db <66282705+allisonwang-db@users.noreply.github.com>
Signed-off-by: Xiao Li <gatorsmile@gmail.com>
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