postgresql update column with least value of column from another table based on condition

I'm trying to run an update query on the column answer_date of a table P. I want to fill each row of answer_date of P with the unique date from create_date column of H where P.ID1 matches with H.ID1 and where P.acceptance_date is not empty.

The query takes a long while to run, so I check the interim changes in answer_date but the entire column is empty like it was created.

Btree indices exists on all the mentioned columns.

Is there something wrong with the query?

UPDATE P
SET    answer_date = subquery.date
FROM (SELECT DISTINCT H.create_date as date
        FROM H, P
        where H.postid=P.acceptance_id
    ) AS subquery

WHERE P.acceptance_id is not null;

Table schema is as follows:

                                              Table "public.P"
        Column         |            Type             | Modifiers | Storage  | Stats target | Description
-----------------------+-----------------------------+-----------+----------+--------------+-------------
 id                    | integer                     | not null  | plain    |              |
 acceptance_id         | integer                     |           | plain    |              |
 answer_date           | timestamp without time zone |           | plain    |              |
Indexes:
    "posts_pkey" PRIMARY KEY, btree (id)
    "posts_accepted_answer_id_idx" btree (acceptance_id) WITH (fillfactor='100')

and

                                     Table "public.H"
      Column       |            Type             | Modifiers | Storage  | Stats target | Description
-------------------+-----------------------------+-----------+----------+--------------+-------------
 id                | integer                     | not null  | plain    |              |
 postid            | integer                     |           | plain    |              |
 create_date       | timestamp without time zone | not null  | plain    |              |
Indexes:
    "H_pkey" PRIMARY KEY, btree (id)
    "ph_creation_date_idx" btree (create_date) WITH (fillfactor='100')

Table P as 70 million rows and H has 220 million rows.

Postgres version is 9.6 Hardware is a Windows laptop with 8Gb of RAM.