MDEV-40245 Match equivalent NULLIF expressions during view analysis

[DerZc]

During view analysis, NULLIF can have two arguments while its fixed form has a third argument for the return value. Generic function comparison rejects these equivalent expressions because their argument counts differ. A view using DISTINCT and ORDER BY can consequently lose distinct rows.

Recognize the two- and three-argument NULLIF forms as equal when both original operands match and the extra argument repeats the first operand.

Regression coverage

Add a seven-line regression to main.view: create one table with two rows, create a view using DISTINCT, ORDER BY and division by NULLIF(1,0), query it once, and clean up. Both distinct values must survive. No extra SET, database creation, manual prepared statements or duplicate queries are needed.

Validation

On current 11.4 at f49e838d367f2154b3edb43effa99ff9eb0d8602:

• The unmodified server fails main.view only on the new query, omitting (NULL, 4.0000).
• The patched Release build succeeds.
• Normal protocol: main.view, main.func_group, main.group_by, main.select and main.func_if pass.
• Prepared-statement protocol: main.view passes.
• View protocol: the same seven-line regression passes as a temporary standalone MTR test. Full main.view is skipped in this mode because its existing count_sessions.inc requires the view protocol to be disabled.
• Expected rows are independently derived from NULLIF(1,0) = 1 and exact decimal division of the distinct input values.
• The full database regression suite was not run.

Bug report: https://jira.mariadb.org/browse/MDEV-40245

[gkodinov]

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

Please simplify the test case.

[gkodinov]

Thanks. LGTM. Please stand by for the final review.