Mysql will still use the column for a covering index and run the query in a single-pass, but it will still generate a temporary table for the results (and do a really slow sort unless you do 'order by null'). It doesn't look like mysql has any way of doing date grouping without temporaries even in the trivial cases. I guess eliminating the sort was enough for my queries.
Using date()-like functions, date_format(), and left() all have identical query plans and roughly comparable performance.
Mysql will still use the column for a covering index and run the query in a single-pass, but it will still generate a temporary table for the results (and do a really slow sort unless you do 'order by null'). It doesn't look like mysql has any way of doing date grouping without temporaries even in the trivial cases. I guess eliminating the sort was enough for my queries.
Using date()-like functions, date_format(), and left() all have identical query plans and roughly comparable performance.