enhance: optimize the memory usage and speed up loading variable length data (#30787)

[longjiquan]

pr: #30787
/kind improvement

[mergify]

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[mergify]

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[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 82.21%. Comparing base (df16bf6) to head (981771f).
Report is 2 commits behind head on 2.3.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.3   #30900      +/-   ##
==========================================
+ Coverage   82.19%   82.21%   +0.02%     
==========================================
  Files         841      831      -10     
  Lines      121457   121377      -80     
==========================================
- Hits        99826    99788      -38     
+ Misses      18402    18358      -44     
- Partials     3229     3231       +2     

Files	Coverage Δ
internal/core/src/common/Json.h	86.27% <100.00%> (+0.85%)	⬆️
internal/core/src/segcore/SegmentSealedImpl.cpp	78.12% <100.00%> (-0.39%)	⬇️
internal/core/src/mmap/Column.h	95.26% <84.61%> (-3.44%)	⬇️

... and 162 files with indirect coverage changes

[czs007]

/approve
/lgtm

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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