Re: Is this GROUP BY query using an index?

[KimSeong Loh]

It is using the index, in fact it reads the index without reading the table data rows. This is shown by the type=index.
No key is selected because, it does not use the index to choose any rows, it read the whole index (whoel table), every single entry in the index.

If you try to remove the index, and do an EXPLAIN, you are likely to see a Using filesort in the extra column, this is required for sorting the rows to do the GROUP BY.