SELECT COUNT(*) FROM large_text_db WHERE X

Where X is something that must be matched exactly. X can be FTS query on FTS-indexed table, but the way COUNT() works in PG is that it's impossible to make it fast. Over large tables, lets say 1B+ rows, it can be very very slow.

Example use case is: searching through a hospital DB of reports that have "pancreatic cancer" in them. This is trivial in SQLite, but in PG it's hard.