Index for date part of field? I have a datetime field, but I often query over the date part, like this 

select * from table where date(datetime_field)=curdate()


This query can't use  index on datetime_field!
What would be the best solution with the minimum amount of changes in code or db?

PS. upgrading mysql could be an option if there is a solution I'm not aware of yet, but a more short term solution for 5.6 would be appreciated
The problem isn't the index.  The problem is that your query as written does not allow an index to be used.

Modify your query so that the date range is resolved to a pair of constants and the problem is solved:

WHERE datetime_field &gt;= CURDATE()
  AND datetime_field &lt; DATE_ADD(CURDATE(), INTERVAL 1 DAY)


This will use the index on the column.

As a rule, never use a column as an argument to a function in WHERE because this forces a table scan.