Here is no sence to use CTE at all.

He's right, read the other note:

Don't forget to convert the percent parameter as a percentage in the body of your function: if the parameter percent is 2 you have to convert it to 0.02.

Very naive of you not to read instructions.

"There are no fractions of people. At the end of each year, the population count is an integer: 252.8 people round down to 252 persons."

OP solved it, closing

If someone want a link to the documentation about full text search and @@ operator: https://www.postgresql.org/docs/current/textsearch-intro.html#TEXTSEARCH-MATCHING

what is the benefit of using rank() vs row_num() ?

I enjoyed the NULLIF inside COALESCE. Congrats!

Rejecting because no random tests, while random tests are already avaialble in similar, C#-only kata here.

From PostgreSQL 14 documentation, 7.2.5:
Currently, window functions always require presorted data, and so the query output will be ordered according to one or another of the window functions' PARTITION BY/ORDER BY clauses. It is not recommended to rely on this, however. Use an explicit top-level ORDER BY clause if you want to be sure the results are sorted in a particular way.
https://www.postgresql.org/docs/14/queries-table-expressions.html

What does the @@ do? I found it on the internet and I'm not 100% sure why it works.

this challenge was so dumb.

I would not consider this code as best practice since it's not capitalising the standard key words (SELECT vs select, FROM vs from and so on ...) -- The indentation also looks rather random then design. I'd consider it best practice if the keywords were in all caps and the indentation with purpose.