How Room Flow queries update

Data and storage Mid-level Kotlin 2.0+ API 24+ Reviewed By Best Latest Apps

Predict the output

A screen observes note 1. Another screen edits note 2. Does the first flow emit?

@Dao
interface NoteDao {
    @Query("SELECT * FROM notes WHERE id = :id")
    fun observe(id: Long): Flow<NoteRow?>

    @Query("UPDATE notes SET text = :text WHERE id = :id")
    suspend fun rename(id: Long, text: String)
}
Options
Hint 1 of 1

What does Room track: rows or tables?

Choose an answer, then open Show answer below to check it.

Show answer

Answer: A Yes, with the same value, because invalidation is per table

Correct. Any write to notes re-runs the query.

Why not the others?
  • B No, because note 1 didn't change. Room doesn't track rows, only tables.
  • C Only if you call refresh(). Room flows update by themselves.
  • D It throws because two screens observe the same table. Many observers are fine.

30-second answer

Room tracks which tables each query reads. When any write touches one of those tables, through Room, it re-runs the query and emits the new result. It's table-level, not row-level, so an update to an unrelated row in the same table also triggers a re-run and an emit, which may be equal to the last result.

Practise saying it · 30 s. No recording, just a timer

Explain how Room flows update.

Say your answer out loud, then check which of these points you covered:

  • Table-level invalidation
  • Re-runs the query on writes
  • Can emit equal results
  • Use distinctUntilChanged

Deep dive

Add distinctUntilChanged() after the DAO flow to drop identical results.

Writes made outside Room, such as raw SQL through another connection, aren't seen.

Joins re-run when any joined table changes.

Keep queries selective and indexed so re-runs stay cheap.

The table used above Compile-checked · Kotlin 2.4.21
@Entity(tableName = "notes")
data class NoteRow(@PrimaryKey val id: Long, val text: String)

Common mistakes

  • Expecting row-level notifications.
  • Mapping heavy work on every emit without distinctUntilChanged.
  • Writing with raw SQLite and wondering why flows don't update.

Senior take & follow-ups

Say 'table-level invalidation'; it explains every surprise.

Follow-up questions

Think of your answer first, then open the hint.

  1. Why might a flow emit the same list twice?

    Show hint

    Another row in the same table changed.

  2. How do you reduce extra work?

    Show hint

    distinctUntilChanged and narrow queries.

Related questions

Suggest a fix if something here is wrong or out of date.