public DPSegment update() throws IOException {
        if (update) {
            return this;
        }
        // Remove outdate info.
        for (int i = 0; i < columnNodes.length; i++) {
            columnNodes[i] = null;
        }
        for (DPColumn col : columns) {
            col.initUpdate();
        }
        update = true;
        return this;
    }
Indicate that this segment can be updated now.
It will initialize the resources like open files, set up caches.
indexr-segment/src/main/java/io/indexr/segment/storage/DPSegment.java