bash, Linux: Set difference between two text files

I have two files A-nodes_to_delete and B-nodes_to_keep. Each file has a many lines with numeric ids. I want to have the list of numeric ids that are in nodes_to_delete but NOT in nodes_to_keep, i.e. A\B Doing it within a PostgreSQL database is unreasonably slow. Any neat way to do it in bash using Linux CLI tools? UPDATE: This would seem to be a Pythonic job, but the files are really, really large

[fcicq]

the awk version is interesting NR==FNR

[takahiroki]

集合の差分を計算する。2 番目の、 sort と uniq だけで実現している answer の方が使い勝手がよい。