MDEV-40748 x NOT IN (<set-operation subquery>) drops rows

[DerZc]

MDEV-40748 x NOT IN () drops rows

An EXCEPT operand can contain a NULL that is removed from the final
result. Track surviving NULLs at the final subquery result and allow the
operands to finish before deciding whether a matching row exists.

Add native regressions for the reported query, equivalent controls, and
repeated prepared-statement execution. Preserve observable results
across the affected execution paths. Update expected output for the
wrapper tests that source the expanded regression.

Bug report: https://jira.mariadb.org/browse/MDEV-40748

Regression coverage

The regression is integrated into the existing main.subselect test. It covers the reported query, equivalent query forms and repeated prepared-statement execution.

Validation

On 11.4 at 322658cd442333a13b46521d1acca67d1e8366cb:

• The unmodified server fails the new regression with a result mismatch.
• The patched server builds successfully.
• 24 query-result checks pass against separately established expected results, including repeated prepared statements.
• Native MTR passes: main.subselect, main.select, main.subselect_no_exists_to_in, main.subselect_no_scache, main.subselect_no_opts, main.subselect_no_semijoin, main.subselect_no_mat.
• The regression passes with the prepared-statement protocol.
• The full database regression suite was not run.

[gkodinov]

Thank you for your contribution! This is a preliminary review.

LGTM. Please stand by for the final review.