What's a pathological case where a bitmap filter would not allow the PROBE(Field, IN-ROW) semijoin reduction optimization?
I figured out the answer to my question - sort of.

When you specify MAXDOP(8), the optimized bitmap filter optimization will create 8 hash buckets.  The exact spread of integers across those hash buckets is probably Pearson hashing.  I am curious if anyone in the comments knows the exact hashing technique, though.

Quoting: https://sqlserverfast.com/blog/hugo/2018/05/plansplaining-part-5-bitmaps/


  Simple example: Let’s say that only three stores in our database have less than 30 employees. These stores have StoreKey 17, 38, and 82. The first row the Bitmap operator receives is for StoreKey 17; it hashes the number 17 and the result is 29, so bit 29 in the bitmap is set to one. It then reads the next row, for StoreKey 38; hashes the number 38 which results in 15; and sets bit 15 to one. The third row it reads is for StoreKey 82; due to a hash collision this number also hashes to 29 so it will once more set bit 29 to one (effectively not changing anything). There are no more rows so the end result will be a bitmap with bits 15 and 29 set to one and all other bits set to zero.