schemadiff: assume default collation for textual column when collation is undefined

[shlomi-noach]

Description

Fixes #15999. When looking at textual columns, if the character set is defined and the collation is not, schemadiff assumes the default collation for that character set.

Related Issue(s)

#15999

cc @lizztheblizz

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 87.50000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 68.25%. Comparing base (3b09eb2) to head (58983d3).
Report is 12 commits behind head on main.

Files	Patch %	Lines
go/vt/schemadiff/column.go	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16000      +/-   ##
==========================================
- Coverage   68.47%   68.25%   -0.23%     
==========================================
  Files        1562     1562              
  Lines      197083   197206     +123     
==========================================
- Hits       134962   134598     -364     
- Misses      62121    62608     +487     

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

[mattlord]

Is this the full comment? 🙂 I'm assuming it should be ~

// This column definition is using the implied / default collation for the character set
// so we need to determine the default/actual collation used for the specified character set
// based on the MySQL version being used. 

[shlomi-noach]

I've updated this and other comments.

[mattlord]

Ah, I see. So "this" and "other" are the two things being diffed. The other related comments are still worthwhile though IMO.

[shlomi-noach]

I've updated this and other comments.