Slow when joining with view

The case
https://stackoverflow.com/questions/30861967/slow-when-joining-with-view

It doesn't make sense that this query took 10*s*:

explain analyzeSELECT x1.id FROM (
SELECT id, data->>'open_course_id' oc_id
FROM student_credits
WHERE (data->>'student_id')::BIGINT = '5500') x1JOIN (
SELECT id FROM V_OpenCourse_WithTC
WHERE semester_id = '53') x2 ON x1.oc_id::BIGINT = x2.id

when both of its components took less than 100*ms*:

SELECT id, data->>'open_course_id' oc_id
FROM student_credits
WHERE (data->>'student_id')::BIGINT = '5500'

SELECT id FROM V_OpenCourse_WithTC
WHERE semester_id = '53'

--
Best Regards,
Kiswono P
GB