unfortunately for now dgraph needs index for all eq, gt and other operations, so it reads all values from that index that are greater than 1512534503, and then uses this list.
issue is described here:
https://github.com/dgraph-io/dgraph/issues/2713
possible workarounds:
use facets. filter by facets doesn’t require index and works as you want. it works but it looks ugly. example is posted in that issue above