Question about composite indexes and WHERE + GROUP BY

Question

Alrighty so I understand how composite indexes work with WHERE but what if a column in the middle of the index is in the GROUP BY, as follows:

CREATE TABLE `test` (
      `a` VARCHAR(255) NOT NULL,
      `b` VARCHAR(255) NOT NULL,
      `c` VARCHAR(255) NOT NULL,
      INDEX(`a`,`b`,`c`)
) ENGINE=InnoDB DEFAULT CHARSET='utf8';

SELECT * FROM test WHERE a = ? AND c = ? GROUP BY b

Would MySQL stop using the index before c, or would it be able to use all three indexes in the above query since b is in the GROUP BY. I haven't been able to find any resources on how exactly GROUP BY utilizes a composite index in a case like the above. In this case I'm using MySQL for database flavor.

Answer 1

The index isn't covering since B isn't part of the WHERE clause as a predicate, so the index is not applicable to this query. In other words, the index stores the data sorted by A then by B then by C, but since you're only filtering on A and C, there's no direct way to seek out C without B. You might end up seeing an index scan or table scan get used instead, in this case.

Answer 2

Start with the = parts of WHERE:

INDEX(a, c,   [to be continued]

Those can be in either order.

Now continue with the GROUP BY:

INDEX(a, c, b)

Based on what you have shown us, that will be covering. (That is, all the needed columns anywhere in the query are in the index.)

See also http://mysql.rjweb.org/doc.php/index_cookbook_mysql