Android Room compile-time warning about column in foreign key not part of an index. What does it mean?

I'm using Android's Room Persistence Library from the Android Architecture Components recently announced at Google I/O. Things seem to be working, but I'm getting the following error:

Warning:tagId column references a foreign key but it is not part of an index. This may trigger full table scans whenever parent table is modified so you are highly advised to create an index that covers this column.

My database has 3 tables: Note, Tag, and JoinNotesTags. Notes to Tags is a many-to-many relationship, hence the JoinNotesTags table to handle the mapping. The tables are straightforward:

• Note.id and Tag.id are both primary keys
• JoinNotesTags.noteId references Note.id
• JoinNotesTags.tagId references Tag.id

The foreign key constraints are defined on the JoinNotesTags table. For reference, here is the CREATE TABLE statement for the JoinNotesTags table:

"CREATE TABLE IF NOT EXISTS `JoinNotesTags` (
    `id` INTEGER PRIMARY KEY AUTOINCREMENT, 
    `noteId` INTEGER, 
    `tagId` INTEGER, 
    FOREIGN KEY(`noteId`) REFERENCES `Note`(`id`) ON UPDATE NO ACTION ON DELETE CASCADE , 
    FOREIGN KEY(`tagId`) REFERENCES `Tag`(`id`) ON UPDATE NO ACTION ON DELETE NO ACTION 
)"

And here is the corresponding @Entity annotation for that class:

@Entity(
        indices = arrayOf(Index(value = *arrayOf("noteId", "tagId"), unique = true)),
        foreignKeys = arrayOf(
                ForeignKey(
                        entity = Note::class,
                        parentColumns = arrayOf("id"),
                        childColumns = arrayOf("noteId"),
                        onDelete = ForeignKey.CASCADE),
                ForeignKey(
                        entity = Tag::class,
                        parentColumns = arrayOf("id"),
                        childColumns = arrayOf("tagId"))
        )
)

As you can see from the @Entity annotation, tagId is included in a composite unique index along with noteId. I've confirmed that this index is correctly defined in the auto-generated json schema file as well:

"CREATE UNIQUE INDEX `index_JoinNotesTags_noteId_tagId` 
    ON `JoinNotesTags` (`noteId`, `tagId`)"

So, my question: Is this warning just a bug in the (still-alpha-release) Room Library -- i.e. the compile-time analysis is missing the fact that tagId is part of this composite index? Or do I really have an indexing problem that I need to resolve in order to avoid full table scans?

Solution 1:

When in kotlin code:

Before

@ColumnInfo(name = "question_id")
var questionId: Long

After

@ColumnInfo(name = "question_id", index = true) //just add index = true
var questionId: Long

Solution 2:

You need to add index to columns for faster queries Here is an example

@Entity(indices = {@Index("artist_id")})
public class Artist{
    @NonNull
    @PrimaryKey
    @ColumnInfo(name = "artist_id")
    public String id;
    public String name;
}

Solution 3:

When you modify the Tag table, the database might need to lookup corresponding rows in the JoinNotesTags table. To be efficient, this requires an index on the tagId column.

Your composite index is not useful for that; because of the way how indexes work, the column(s) to be searched must be the leftmost column(s) in the index.

You should add an index on only the tagId column. (You could swap the order of the columns in the composite index, but then you'd have the same problem with noteId.)