refactor(plan): migrate remaining legacy UPDATE special cases

[ck89119]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #26342

What this PR does / why we need it:

This PR removes the remaining legacy UPDATE planner fallbacks owned by issue #26342.

• Route primary-key and unique-key updates on mo_pubs and mo_subs through the modern MULTI_UPDATE planner.
• Route legal SET ... AUTO_INCREMENT updates through MULTI_UPDATE while preserving SET value conversion and PRE_INSERT handling.
• Keep auto-increment foreign-key updates on their existing legacy route, with a precise auto_increment_foreign_key telemetry reason.
• Add planner regressions for pub/sub keys, SET conversion, PRE_INSERT, MULTI_UPDATE, and the retained foreign-key control path.

Validation:

• Focused UPDATE planner tests.
• Full pkg/sql/plan tests with coverage.
• Full dependent pkg/sql/compile tests.
• go build and go vet for pkg/sql/plan and pkg/sql/compile.
• git diff --check.

[qodo-code-review]

Qodo reviews are paused for this user.

Troubleshooting steps vary by plan Learn more →

On a Teams plan?
Reviews resume once this user has a paid seat and their Git account is linked in Qodo.
Link Git account →

Using GitHub Enterprise Server, GitLab Self-Managed, or Bitbucket Data Center?
These require an Enterprise plan - Contact us
Contact us →

[XuPeng-SH]

Deep-reviewed at cb39c93. No blocking findings.

Checked the removed legacy routes end to end:

• mo_pubs/mo_subs PK/UK updates now build modern MULTI_UPDATE contexts, including the mo_subs hidden unique index; the integration test covers key migration, duplicate-key atomicity, ordinary/join updates, and tenant authorization.
• SET AUTO_INCREMENT preserves SET conversion and PRE_INSERT. On an exact-head temporary service I compared explicit, 0, NULL, and DEFAULT assignments against the base legacy path; plans changed as intended and row results remained identical.
• The remaining auto-increment + affected-FK case keeps an explicit typed legacy reason instead of becoming a catch-all.

Exact-head validation passed: full pkg/sql/plan and pkg/sql/compile tests, TestIssue26342MoSubsModernUpdate, go vet for pkg/sql/plan and pkg/tests/issues, and a full mo-service build.

[ck89119]

@Mergifyio refresh

[mergify]

refresh

✅ Pull request refreshed

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-08-03 07:17 UTC · Rule: main · triggered by rule Automatic queue on approval for main
• 🟠 Checks running · in-place
• 🚫 Left the queue — 2026-08-03 08:09 UTC · at 72a21bb86b5abb036b4477345f3821edaf2fc092

This pull request spent 51 minutes 34 seconds in the queue, with no time running CI.

Reason

Pull request #26596 has been merged manually at 1878a38

Hint

You were too fast!

Tick the box to put this pull request back in the merge queue (same as @mergifyio queue).

• Requeue this pull request