See the attached patch, which recomputes the count regularly. I don't claim the patch is committable or that it has no other bugs, but hopefully it shows what I meant.
I got only one issue it is not work well with cursor
postgres=# START TRANSACTION;
START TRANSACTION
postgres=# create table t as select i from generate_series(1,1000) s(i);
SELECT 1000
postgres=# declare c cursor for select * from t fetch first 5 percent rows only;
DECLARE CURSOR
postgres=# fetch all in c;
ERROR: trying to store a minimal tuple into wrong type of slot
I am looking at it .
meanwhile i fix row estimation and cost and make create_ordered_paths creation with no LIMIT consideration in PERCENTAGE case
