Re: bug (5.6.22) in ADDTIME?

[Rick James]

Here's another user's opinion. GROUP BY is tricky, and there have been changes to the optimizer that indirectly affect the resultset you get.

Here's another variant on it:
https://mariadb.com/kb/en/mariadb/group-by-trick-has-been-optimized-away/

Bottom line: In this
SELECT x, y ... GROUP BY x
you should _not_ depend on which `y` you get.

Any of these _may_ cause the result set to change:
DELETE a row
UPDATE a row
OPTIMIZE TABLE
ANALYZE TABLE
ALTER TABLE
Change the ENGINE

Similarly, in
SELECT ... LIMIT 1 -- without an ORDER BY
One day you get one row; the next day you _might_ get different row.