Codecov Report

Attention: Patch coverage is 98.76543% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.80%. Comparing base (119d11a) to head (9033e4c).
Report is 10 commits behind head on master.

@@            Coverage Diff             @@
##           master    #6322      +/-   ##
==========================================
- Coverage   88.90%   88.80%   -0.11%     
==========================================
  Files         241      246       +5     
  Lines       40793    41143     +350     
  Branches     3435     3483      +48     
==========================================
+ Hits        36266    36536     +270     
- Misses       4491     4564      +73     
- Partials       36       43       +7     

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I cannot think now of a better way, but these multiple level loop seems hard to follow. Isn't there a way to make the loop over AgreeOnId more explicit?

There is no need for IT_V2? I think

I renamed the constructor NewIntersectionIterator instead of NewIntersectIterator to be more aligned with NewUnionIterator and more grammatically correct, so the IT_V2 macro is not required

I cannot think now of a better way, but these multiple level loop seems hard to follow. Isn't there a way to make the loop over AgreeOnId more explicit?

I don't think so, and it's better to maintain the current separation due to the differences in the outer loop when validation is required.
We must have 2 loops for the core logic

while (no agreement) {
    for (every child) {
        check if the current child has the current target ID
    }
}