Android's count field in BaseColums

i have been working with a database defining the columns with the help of the BaseColumns inter开发者_JS百科face. its pretty standard implementation. but i am curious about the Count constant mentioned there. i am using a content provider for the queries so a custom count query isn't really an option. so far i have been using the query count with a projection of only the ids, seemed more lighweight that way.

what is the _COUNT field in BaseColumns used for? i am assuming its for a count of rows. how do i build the scheme with that field?

how do i query the count?