How to use row_number in a where clause

Question

I'm trying to use window functions to get the most recent n records, following from here:

I have:

select 
   id,
   blah,
   row_number () over (
     partition by blah, my_id
     order by datetime) rn,
   theme
from documents
where theme = 'cats';

And I get:

 id | blah | rn | theme 
----+-----+----+-------
  1 |   1 |  1 | cats
  2 |   1 |  2 | cats
  3 |   1 |  3 | cats
  4 |   1 |  4 | cats
  5 |   1 |  5 | cats
  9 |   2 |  1 | cats
  8 |   2 |  2 | cats
 11 |   3 |  1 | cats
 12 |   4 |  1 | cats
 13 |   5 |  1 | cats
 14 |   6 |  1 | cats
(11 rows)

Which is great. But I want not more than 2 rows, rn <= 2, for example. I imagine this as something like:

select 
   id,
   blah,
   row_number () over (
     partition by blah, my_id
     order by datetime) rn,
   theme
from documents
where theme = 'cats' and
rn <= 2;

but I get:

ERROR:  column "rn" does not exist
LINE 15: rn <= 1;
         ^

I know I can make this a subquery like the linked question, but there must be syntax I'm missing for putting the row_number in the where clause, right? What is it?

Answer 1

You need a derived table:

select id, blah, them
from (
  select id,
         blah,
         row_number () over (partition by blah, my_id order by datetime) rn,
         theme
  from documents
) x
where theme = 'cats' 
and rn <= 2;

This is basically syntactic sugar and does not cause a performance overhead.