The meaning/intent of the following methods - JSON storage format - part 1

[Edmon Begoli]

I am studying the JSON Storage plugin.

Can someone please answer what is the meaning (or intent) of the following
methods:

# Question 1:
# This is from inside the getter for RecordWriter. What is major/minor
fragment id?
String fragmentId = String.format("%d_%d", handle.getMajorFragmentId(),
handle.getMinorFragmentId());


# Question 2:
# Speficially, what is Operator Type, and what is JSON specific about it?
public int getReaderOperatorType() {
    return CoreOperatorType.JSON_SUB_SCAN_VALUE;
  }

# Question 3:
# same, but for writer
  @Override
  public int getWriterOperatorType() {
    throw new UnsupportedOperationException();
  }

# Question 4:
# I generally understand the concept of predicate pushdown, but is this
about predicate push down or something else?
# How does one implement a push down for storage format?
  @Override
  public boolean supportsPushDown() {

# Question 5:
# Is this int value of 31 purely randomly selected, or is there an index
somewhere?
  @Override
  public int hashCode() {
      return 31;
  }