presto-main/src/main/java/io/prestosql/operator/aggregation/KeyValuePairs.java/KeyValuePairs/keyExists
class KeyValuePairs:    private boolean keyExists(Block key, int position)
    {
        checkArgument(position >= 0, "position is negative");
        return keyPositionByHash[getHashPositionOfKey(key, position)] != EMPTY_SLOT;
    }