vdiff: do not sort by table name in summary, it is not necessary

[nickvanw]

Description

Removes the unnecessary ORDER BY table_name clause from the VDiff summary query. This ordering was causing MySQL to perform a filesort on potentially large JSON report blobs, leading to "Out of sort memory" errors when VDiffs involve many tables. The sort provided no value since the results are stored in Go maps which have non-deterministic iteration order anyway.

I'm backporting this as it can effectively break certain VDiff commands without reconfiguring MySQL unnecessarily.

Related Issue(s)

• Fixes: VDiff summary query can fail with 'Out of sort memory' error #18973

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

AI Disclosure

AI was not used to remove the ORDER BY clause, though it's opinion was asked on whether that was a safe change to make, as well as to generate the commit message.

[vitess-bot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.77%. Comparing base (1f49de4) to head (c93a71c).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18972      +/-   ##
==========================================
- Coverage   69.77%   69.77%   -0.01%     
==========================================
  Files        1608     1608              
  Lines      214908   214942      +34     
==========================================
+ Hits       149953   149967      +14     
- Misses      64955    64975      +20     

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[mattlord]

LGTM! Thanks, @nickvanw! ❤️

[timvaillancourt]

Makes sense to me, nice catch! 🚀