understanding collapsingQParser with facet vs group.facet

[Derek Poh]

Hi

I read about "collapsingQParser returns the facet count the same as 
group.truncate=true" and has this issue with the facet count and the 
after filter facet count notthe same.
Using group.facetdoes not has this issue but it's performance is very 
badcompared to collapsingQParser.

I trying to understand why collapsingQParser behave this way and will 
need to explain to management.

Can someone explain how collapsingQParser calculatethefacet 
countscompated to group.facet?

Thank you,
Derek