Most efficient way to increment a value of everything in Firebase

FOR FIRESTORE API ONLY, NOT FIREBASE

Thanks to the latest Firestore patch (March 13, 2019), you don't need to follow the other answers above.

Firestore's FieldValue class now hosts a increment method that atomically updates a numeric document field in the firestore database. You can use this FieldValue sentinel with either set (with mergeOptions true) or update methods of the DocumentReference object.

The usage is as follows (from the official docs, this is all there is):

DocumentReference washingtonRef = db.collection("cities").document("DC");

// Atomically increment the population of the city by 50.
washingtonRef.update("population", FieldValue.increment(50));

If you're wondering, it's available from version 18.2.0 of firestore. For your convenience, the Gradle dependency configuration is implementation 'com.google.firebase:firebase-firestore:18.2.0'

Note: Increment operations are useful for implementing counters, but keep in mind that you can update a single document only once per second. If you need to update your counter above this rate, see the Distributed counters page.

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EDIT 1: FieldValue.increment() is purely "server" side (happens in firestore), so you don't need to expose the current value to the client(s).

EDIT 2: While using the admin APIs, you can use admin.firestore.FieldValue.increment(1) for the same functionality. Thanks to @Jabir Ishaq for voluntarily letting me know about the undocumented feature. :)

EDIT 3:If the target field which you want to increment/decrement is not a number or does not exist, the increment method sets the value to the current value! This is helpful when you are creating a document for the first time.

This is one way to loop over all items and increase their priority:

var estimatesRef = firebase.child('Estimates');
estimatesRef.once('value', function(estimatesSnapshot) {
  estimatesSnapshot.forEach(function(estimateSnapshot) {
    estimateSnapshot.ref().update({
      estimateSnapshot.val().priority + 1
    });
  });
});

It loops over all children of Estimates and increases the priority of each.

You can also combine the calls into a single update() call:

var estimatesRef = firebase.child('Estimates');
estimatesRef.once('value', function(estimatesSnapshot) {
  var updates = {};
  estimatesSnapshot.forEach(function(estimateSnapshot) {
    updates[estimateSnapshot.key+'/priority'] = estimateSnapshot.val().priority + 1;
  });
  estimatesRef.update(updates);
});

The performance will be similar to the first solution (Firebase is very efficient when it comes to handling multiple requests). But in the second case it will be sent a single command to the server, so it will either fail or succeed completely.