chore: refine plan id generation

[leiysky]

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Summary

Generate plan id in top-down sequence, which means the parent plans will have smaller plan id.

Before this PR:

Join_2
    Scan_0
    Scan_1

With this PR:

Join_0
    Scan_1
    Scan_2

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is