I see there's a couple MCV-specific functions in the extended_stats.c. Shouldn't those go into mvc.c instead?
I wanted to put it there, but there was a reason I didn't and I've now forgotten what it was. I'll make an effort to relocate it to mcv.c.
Looking at it now, I see that check_mcvlist_array() expects access to extarginfo, so I either I add a bunch of Datum-aware and extarginfo-aware code to mcv.c, or I do those checks outside of import_mcvlist() and leave check_mcvlist_array() where it is, which is my current inclination, though obviously it's not a perfectly clean break.