Server-side count of keys for FoundationDB. Still has to scan the complete database. Modified with input from the FDB team to use getKey.

FDBCount.java

      // 1) Stride through the database using KeySelectors until you pass the end of the database
      // 2) Then, back off until you find the last key
      final AtomicBoolean retry = new AtomicBoolean();
      Function<Transaction, Long> function = new Function<Transaction, Long>() {
        private long start = System.currentTimeMillis();
        private long count = 0;
        private int offset = 1000000;
        private KeySelector keySelector = KeySelector.firstGreaterOrEqual(new byte[0]);
        private boolean narrowing = false;

        @Override
        public Long apply(Transaction transaction) {
          // Just in case it tries to load the data
          transaction.options().setReadYourWritesDisable();
          if (retry.get()) {
            offset = Math.max(1, offset /= 2);
            System.out.println("Retry, reducing offset size to: " + offset);
          } else {
            retry.set(true);
          }
          byte[] key = transaction.getKey(keySelector.add(offset)).get();
          if (isSystemKey(key)) {
            offset = Math.max(1, offset /= 2);
            narrowing = true;
          } else {
            keySelector = KeySelector.firstGreaterThan(key);
            count += offset;
            if (narrowing) {
              key = transaction.getKey(keySelector).get();
              if (isSystemKey(key)) {
                return count;
              }
            }
          }
          System.out.println(count + " @ " + (1000l * count / (System.currentTimeMillis() - start)) + " per second");
          return null;
        }

        private boolean isSystemKey(byte[] key) {
          return key.length == 1 && key[0] == -1;
        }
      };
      Long count;
      while ((count = database.run(function)) == null) {
        retry.set(false);
      }
      System.out.println(count + " keys");