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For one of observables (single, maybe) we don't specify a subscvription thread so that developer can control where the action happens.
For flowables / observables; we always use the Room IO thread fo rthe actual query since we may repeat the work.

For LiveData, ti si always the Room IO Thread.

Until we can extend Room Query methods to receive non-query parameter (e.g. a Scheduler) we cannot do this.
At least room catches this instantly since it will crash w/ db access on main thread exception.

Hello Yigit, nice to meet you here! :)
Probably I should have described the problem more clearly.

I'm using Flowable, and hopefully it executes queries in background thread (at least, app does not crash with IllegalStateException, as with Single).

But nevertheless subscribing and disposing(!) does cause StrictMode violations. Stack trace examples:

D/StrictMode: StrictMode policy violation; ~duration=17 ms: android.os.StrictMode$StrictModeDiskWriteViolation: policy=31 violation=1
        at android.os.StrictMode$AndroidBlockGuardPolicy.onWriteToDisk(StrictMode.java:1111)
        at android.database.sqlite.SQLiteConnection.applyBlockGuardPolicy(SQLiteConnection.java:1043)
        at android.database.sqlite.SQLiteConnection.execute(SQLiteConnection.java:552)
        at android.database.sqlite.SQLiteSession.beginTransactionUnchecked(SQLiteSession.java:323)
        at android.database.sqlite.SQLiteSession.beginTransaction(SQLiteSession.java:298)
        at android.database.sqlite.SQLiteDatabase.beginTransaction(SQLiteDatabase.java:505)
        at android.database.sqlite.SQLiteDatabase.beginTransaction(SQLiteDatabase.java:416)
        at androidx.sqlite.db.framework.FrameworkSQLiteDatabase.beginTransaction(FrameworkSQLiteDatabase.java:69)
        at androidx.room.InvalidationTracker.syncTriggers(InvalidationTracker.java:426)
        at androidx.room.InvalidationTracker.syncTriggers(InvalidationTracker.java:465)
        at androidx.room.InvalidationTracker.addObserver(InvalidationTracker.java:249)
        at androidx.room.RxRoom$1.subscribe(RxRoom.java:79)
        at io.reactivex.internal.operators.flowable.FlowableCreate.subscribeActual(FlowableCreate.java:71)
        at io.reactivex.Flowable.subscribe(Flowable.java:14479)
        at io.reactivex.internal.operators.flowable.FlowableObserveOn.subscribeActual(FlowableObserveOn.java:56)
        at io.reactivex.Flowable.subscribe(Flowable.java:14479)
        at io.reactivex.internal.operators.flowable.FlowableFlatMapMaybe.subscribeActual(FlowableFlatMapMaybe.java:54)
        at io.reactivex.Flowable.subscribe(Flowable.java:14479)
        at io.reactivex.Flowable.subscribe(Flowable.java:14416)
        at io.reactivex.Flowable.subscribe(Flowable.java:14306)
        at gmk57.strictroom.MainActivity.onCreate(MainActivity.java:30)

StrictMode policy violation; ~duration=108 ms: android.os.StrictMode$StrictModeDiskWriteViolation: policy=31 violation=1
        at android.os.StrictMode$AndroidBlockGuardPolicy.onWriteToDisk(StrictMode.java:1111)
        at android.database.sqlite.SQLiteConnection.applyBlockGuardPolicy(SQLiteConnection.java:1043)
        at android.database.sqlite.SQLiteConnection.execute(SQLiteConnection.java:552)
        at android.database.sqlite.SQLiteSession.beginTransactionUnchecked(SQLiteSession.java:323)
        at android.database.sqlite.SQLiteSession.beginTransaction(SQLiteSession.java:298)
        at android.database.sqlite.SQLiteDatabase.beginTransaction(SQLiteDatabase.java:505)
        at android.database.sqlite.SQLiteDatabase.beginTransaction(SQLiteDatabase.java:416)
        at androidx.sqlite.db.framework.FrameworkSQLiteDatabase.beginTransaction(FrameworkSQLiteDatabase.java:69)
        at androidx.room.InvalidationTracker.syncTriggers(InvalidationTracker.java:426)
        at androidx.room.InvalidationTracker.syncTriggers(InvalidationTracker.java:465)
        at androidx.room.InvalidationTracker.removeObserver(InvalidationTracker.java:281)
        at androidx.room.RxRoom$1$2.run(RxRoom.java:83)
        at io.reactivex.disposables.ActionDisposable.onDisposed(ActionDisposable.java:30)
        at io.reactivex.disposables.ActionDisposable.onDisposed(ActionDisposable.java:19)
        at io.reactivex.disposables.ReferenceDisposable.dispose(ReferenceDisposable.java:43)
        at io.reactivex.internal.disposables.DisposableHelper.dispose(DisposableHelper.java:124)
        at io.reactivex.internal.disposables.SequentialDisposable.dispose(SequentialDisposable.java:72)
        at io.reactivex.internal.operators.flowable.FlowableCreate$BaseEmitter.cancel(FlowableCreate.java:301)
        at io.reactivex.internal.operators.flowable.FlowableObserveOn$BaseObserveOnSubscriber.cancel(FlowableObserveOn.java:154)
        at io.reactivex.internal.operators.flowable.FlowableFlatMapMaybe$FlatMapMaybeSubscriber.cancel(FlowableFlatMapMaybe.java:158)
        at io.reactivex.internal.subscriptions.SubscriptionHelper.cancel(SubscriptionHelper.java:189)
        at io.reactivex.internal.subscribers.LambdaSubscriber.cancel(LambdaSubscriber.java:119)
        at io.reactivex.internal.subscribers.LambdaSubscriber.dispose(LambdaSubscriber.java:104)
        at gmk57.strictroom.MainActivity.onDestroy(MainActivity.java:37)

oh this is bad :/ we should fix it, thanks!

Looks like it is broken for both add observer and remove observer since both are sync.
We need to investigate this better because these methods were not @WorkerThread before and we had to change them to fix some invalidation problems (see internal bug b/73592149).
Luckily this is not an issue for LiveData since it uses ComputableLiveData and always calls observe on a background thread.

Project: platform/frameworks/support
Branch: androidx-master-dev

commit 79deed62f2188a6ce2d21244dd28c3dc8ab6d0de
Author: Daniel Santiago Rivera <danysantiago@google.com>
Date:   Wed Oct 17 11:07:39 2018

    Specify subscribeOn and unsubscribeOn scheduler in RxRoom.
   
    Setting subscriber and unsubscriber schedulers prevents triggers being
    synced in the main thread when subscribing or disposing a RxRoom
    flowable / observable.
   
    Moved beginTransaction() calls in InvalidationTracker outside the try,
    finally with endTransaction() so that exceptions thrown by
    beginTransaction() aren't aborted nor forgotten.
   
    Bug: 117201279
    Test: Manual test with sample app.
    Change-Id: Iefb33dc63cf78e125a8805bd66a26ada365d8453

A       room/integration-tests/testapp/src/androidTest/java/androidx/room/integration/testapp/test/InvalidationTrackerTest.java
M       room/runtime/src/main/java/androidx/room/InvalidationTracker.java
M       room/rxjava2/src/main/java/androidx/room/RxRoom.java
M       room/rxjava2/src/test/java/androidx/room/RxRoomTest.java

https://android-review.googlesource.com/792524
https://goto.google.com/android-sha1/79deed62f2188a6ce2d21244dd28c3dc8ab6d0de

Dupe https://issuetracker.google.com/issues/74227569

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