afl_filter is currently a varchar(64) binary NOT NULL field. This is because, since global filters have been introduced, that field could hold either a plain numeric ID or the string "global-ID", where ID is again a numeric identifier. The alter was done in patch-global_filters.sql introduced in rEABF14b850f891de27ea09a1439e3835f66c49ad773f, and it was a poor choice.
It denormalized more the structure of the DB, and the code was never properly changed to handle afl_filter as a string. In fact, until yesterday there were 4 JOINs on af_id=afl_filter, which is a join between an integer and a string. That syntax isn't even supported in postgres, and at any rate, it slows down the query a bit because every row has to be implicitly cast before the comparison.
So basically we've lived with a stopgap in place for 10 years, and it's time to get rid of it. The proposal (already implemented) is to split the afl_filter in
afl_filter_id BIGINT unsigned NOT NULL, afl_global tinyint(1) NOT NULL
and clean up the code accordingly. This also has the benefit of encouraging the integer type for filter IDs all around the code, since we won't need string IDs anymore.