ddl: Fix invalid expression index

[JaySon-Huang]

What problem does this PR solve?

Issue Number: close #61133

Problem Summary:
TiDB accept this kind of invalid expression index. If tiflash replica is created and new rows inserted, it cause TiFlash crashes becuase unexpectedly need to create a "Nothing" data type in the storage layer. pingcap/tiflash#9891

TiDB [email protected]:test> CREATE TABLE `t2` (
                        ->   `id` int,
                        ->   KEY `idx_name` ((null))
                        -> );
TiDB [email protected]:test> show create table t2 \G
***************************[ 1. row ]***************************
Table        | t2
Create Table | CREATE TABLE `t2` (
  `id` int DEFAULT NULL,
  KEY `idx_name` ((null))

What changed and how does it work?

For a valid expression, the return value type stored in generated column should never be TypeNull. Add a check whether col.FieldType.GetType() == mysql.TypeNull is true. If it is true, then refuse to create the expression index.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix a bug that invalid expression index could be created

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.2518%. Comparing base (da5de78) to head (3dea9d1).
Report is 7 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #61135        +/-   ##
================================================
+ Coverage   73.1620%   75.2518%   +2.0898%     
================================================
  Files          1724       1746        +22     
  Lines        477317     489003     +11686     
================================================
+ Hits         349215     367984     +18769     
+ Misses       106687      98140      -8547     
- Partials      21415      22879      +1464     

Flag	Coverage Δ
integration	49.3963% <100.0000%> (?)
unit	72.7842% <0.0000%> (+0.4065%)	⬆️

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Components	Coverage Δ
dumpling	52.7804% <ø> (ø)
parser	∅ <ø> (∅)
br	62.6206% <ø> (+14.8793%)	⬆️

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[JaySon-Huang]

/test check-dev2

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lance6716, wjhuang2016

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-05-15 10:27:06.844560293 +0000 UTC m=+21993.100457687: ☑️ agreed by wjhuang2016.
• 2025-05-16 07:51:13.658427139 +0000 UTC m=+99039.914324535: ☑️ agreed by lance6716.

[ti-chi-bot]

@JaySon-Huang: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/check_dev	3dea9d1	link	unknown	/test check-dev

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