schemadiff small internal refactor: formalizing column charset/collation

[shlomi-noach]

Description

This PR does some internal refactoring/cleanup without changing behavior. We give ColumnDefinitionEntity some more brainpower, cleanup column diffing (introducing SetExplicitCharsetCollate and simplifying ColumnDiff().

Related Issue(s)

As part of #16229

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

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 81.48148% with 10 lines in your changes missing coverage. Please review.

Project coverage is 68.72%. Comparing base (7a737f4) to head (e3232fb).
Report is 32 commits behind head on main.

Files	Patch %	Lines
go/vt/schemadiff/table.go	33.33%	6 Missing ⚠️
go/vt/schemadiff/column.go	91.11%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16239      +/-   ##
==========================================
+ Coverage   68.61%   68.72%   +0.11%     
==========================================
  Files        1544     1547       +3     
  Lines      197993   198274     +281     
==========================================
+ Hits       135848   136266     +418     
+ Misses      62145    62008     -137     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[rohit-nayak-ps]

I don't see where this is used ...

[shlomi-noach]

You're right. This is preparation work for a later PR (WIP: #16256).

[mattlord]

Only one minor comment. There's no new tests added here, but I'm assuming that the behavior is covered by the existing tests, correct?

[mattlord]

Why do this when we blindly set c.columnDefinition.Type.Options.Collate = c.tableCharsetCollate.collate in the subsequent IF condition?

[shlomi-noach]

Good catch! Removed.

[shlomi-noach]

There's no new tests added here, but I'm assuming that the behavior is covered by the existing tests, correct?

That's right. It's an internal refactor with no new behavioral change, hence covered by existing tests.