vtgate: Reject unqualified * after comma in SELECT list

[arthurschreiber]

Description

MySQL only allows unqualified * as the first expression in a SELECT list. After a comma, only qualified table.* is permitted (e.g. SELECT a, t.* FROM t is fine, but SELECT a, * FROM t is a syntax error). The Vitess parser accepted * in any position, diverging from MySQL behavior.

This adds a check in the select_expression_list grammar rule to reject unqualified * when it appears after a comma.

MySQL behavior reference:

Query	MySQL Result
SELECT * FROM t	OK
SELECT *, a FROM t	OK
SELECT a, t.* FROM t	OK
SELECT t.*, t.* FROM t	OK
SELECT a, * FROM t	ERROR 1064
SELECT *, * FROM t	ERROR 1064

Note

This means that some queries that worked in Vitess previously might fail after this change.

This is only relevant for queries targeting tables in sharded keyspaces due to the automatic expansion of unqualified * expressions.

Queries against unsharded keyspaces would have failed even before this fix once they reached MySQL.

Related Issue(s)

N/A

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

N/A

AI Disclosure

Most of this PR was written by Claude Code.

[vitess-bot]

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[promptless]

📝 Documentation updates detected!

New suggestion: Document SELECT expression syntax rules for * wildcard

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Tip: Tag @Promptless in GitHub PR comments to guide documentation changes during code review 🐙

[arthurschreiber]

I dismissed Promptless' documentation suggestion, because I don't think we need to document behavior where Vitess and MySQL match (because that should be the "default" behavior).