Fixes around derived table planning

[systay]

Description

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.41%. Comparing base (da1301b) to head (2f338ca).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15688      +/-   ##
==========================================
+ Coverage   68.40%   68.41%   +0.01%     
==========================================
  Files        1556     1556              
  Lines      195418   195420       +2     
==========================================
+ Hits       133666   133698      +32     
+ Misses      61752    61722      -30     

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[systay]

moved to #15588