// To add an index to all collections in the database that start with "test" and have a field named "ts", and to run the operation in the background, use the following command:

db.getCollectionNames().forEach(function(collname) {
  if (collname.startsWith("test")) {
    db.getCollection(collname).createIndex({ts: 1}, {background: true});
  }
});

db.getCollectionNames().forEach(function(collname) {
  if (collname.startsWith("test")) {
    if (!db.getCollection(collname).getIndexes().some(function(index) { return index.name === "ts_1"; })) {
      db.getCollection(collname).createIndex({ts: 1}, {background: true});
    }
  }
});