presto-spi/src/main/java/io/prestosql/spi/block/MapBlock.java/MapBlock/calculateSize
class MapBlock:    private long calculateSize(Block block)
    {
        int entriesStart = offsets[startOffset];
        int entriesEnd = offsets[startOffset + positionCount];
        int entryCount = entriesEnd - entriesStart;
        return block.getRegionSizeInBytes(entriesStart, entryCount);
    }