RewriteQueriesToDropUnusedColumns

When present, RewriteQueriesToDropUnusedColumns annotation will cause Room to rewrite your Query methods such that only the columns that are used in the response are queried from the database.

This annotation is useful if you don't need all columns returned in a query but also don't want to spell out their names in the query projection.

For example, if you have a User class with 10 fields and want to return only the name and lastName fields in a POJO, you could write the query like this:

@Dao
interface MyDao {
    @Query("SELECT * FROM User")
    public fun getAll(): List<NameAndLastName>
}

data class NameAndLastName (
    val name: String,
    val lastName: String
)

Normally, Room would print a RoomWarnings.QUERY_MISMATCH warning since the query result has additional columns that are not used in the response. You can annotate the method with RewriteQueriesToDropUnusedColumns to inform Room to rewrite your query at compile time to avoid fetching extra columns.

@Dao
interface MyDao {
    @RewriteQueriesToDropUnusedColumns
    @Query("SELECT * FROM User")
    fun getAll(): List<NameAndLastName>
}

At compile time, Room will convert this query to SELECT name, lastName FROM (SELECT * FROM User) which gets flattened by Sqlite to SELECT name, lastName FROM User.

When the annotation is used on a Dao method annotated with Query, it will only affect that query. You can put the annotation on the Dao annotated class/interface or the Database annotated class where it will impact all methods in the dao / database respectively.

Note that Room will not rewrite the query if it has multiple columns that have the same name as it does not yet have a way to distinguish which one is necessary.

Summary

RewriteQueriesToDropUnusedColumns()