Postgres SQL - performance issue with boolean field

In my table plin_korisnik, I have field active which is defined as boolean type.

I'm trying to execute this query to fetch data from that table and two other tables:

SELECT
    pk.omm AS omm,
    pk.br_plin AS br_plin,
    pk.naziv AS naziv,
    pk.ulica||' '||pk.kbr AS adresa,
    pk.pu||' - '||pk.naziv_pu AS mjesto,
    po.datum AS datum,
    CASE WHEN po.stanje >= 999999 THEN NULL ELSE po.stanje END AS stanje,
    po.napomena AS napomena,
    po.plin_postar AS laus,
    pp.ime||' '||pp.prezime AS postar
FROM plin_korisnik pk
INNER JOIN
    plin_ocitanje po ON pk.omm = po.omm
INNER JOIN plin_postar pp ON pp.laus = po.plin_postar
WHERE po.datum>='2017-01-26'
    AND po.datum<='2017-01-26'
    AND pk.tip='p'
    AND pk.active = TRUE
ORDER BY po.datum, pk.naziv

but query takes to much time (like forever; I interrupted it after half an hour), but when I remove pk.active = TRUE test from WHERE clause, then query executes with expected speed. I had try to cast boolean type to integer, but problem remains.

I'd appreciate it if someone could explain how to use a boolean field in this and similar queries. active field is not indexed, maybe it should be, please help.

EDIT: After few hours of thinking I came out with this solution which use WITH clause:

WITH pk AS (
    SELECT * FROM plin_korisnik WHERE active AND tip='p' 
)
SELECT
   pk.omm AS omm,
   pk.br_plin AS br_plin,
   pk.naziv AS naziv,
   pk.ulica||' '||pk.kbr AS adresa,
   pk.pu||' - '||pk.naziv_pu AS mjesto,
   po.datum AS datum,
   CASE WHEN po.stanje >= 999999 THEN NULL ELSE po.stanje END AS stanje,
   po.napomena AS napomena,
   po.plin_postar AS laus,
   pp.ime||' '||pp.prezime AS postar
FROM pk
INNER JOIN
   plin_ocitanje po ON pk.omm = po.omm
INNER JOIN plin_postar pp ON pp.laus = po.plin_postar
WHERE po.datum>='2017-01-26'
   AND po.datum<='2017-01-26'
ORDER BY po.datum, pk.naziv;