fix: avoid unnecessary get_fragments calling during plan compaction

[zhangyue19921010]

Avoid unnecessary get_fragments calling during plan compaction

get_fragments involves deep copying of dataset and fragment, which is pretty heavy.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.84%. Comparing base (d8f9dcb) to head (e10541b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5179   +/-   ##
=======================================
  Coverage   81.83%   81.84%           
=======================================
  Files         341      341           
  Lines      140682   140691    +9     
  Branches   140682   140691    +9     
=======================================
+ Hits       115122   115142   +20     
+ Misses      21752    21742   -10     
+ Partials     3808     3807    -1     

Flag	Coverage Δ
unittests	81.84% <100.00%> (+<0.01%)	⬆️

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[Xuanwo]

Thank you!