planner: only execute GetExplainRowsForPlan when we need it

[Reminiscent]

What problem does this PR solve?

Issue Number: close #30360

Problem Summary:
GetExplainRowsForPlan called twice for the same query

What is changed and how it works?

only execute GetExplainRowsForPlan when we need it

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

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