How to speed up the query in PostgreSQL

I have DB in PostgreSQL with a big data (now it is somewhere around 46 GB and the db will keep growing). I created indexes on often used columns and adapted the config file:

shared_buffers = 1GB
temp_buffers = 256MB
work_mem = 512MB

But this query is still slow:

select distinct us_category_id as cat, count(h_user_id) as res from web_hits 
inner join users on h_user_id = us_id 
where (h_datetime)::date = ('2015-06-26')::date and us_category_id != ''
group by us_category_id

Explain Analyze:

HashAggregate (cost=2870958.72..2870958.93 rows=21 width=9) (actual time=899141.683..899141.683 rows=0 loops=1)

Group Key: users.us_category_id, count(web_hits.h_user_id)
-> HashAggregate (cost=2870958.41..2870958.62 rows=21 width=9) (actual time=899141.681..899141.681 rows=0 loops=1)

Group Key: users.us_category_id
 -> Hash Join (cost=5974.98..2869632.11 rows=265259 width=9) (actual time=899141.679..899141.679 rows=0 loops=1)

Hash Cond: ((web_hits.h_user_id)::text = (users.us_id)::text)
-> Seq Scan on web_hits (cost=0.00..2857563.80 rows=275260 width=7) (actual time=899141.676..899141.676 rows=0 loops=1)
-> Seq Scan on web_hits (cost=0.00..2857563.80 rows=275260 width=7) (actual time=899141.676..899141.676 rows=0 loops=1)
Filter: ((h_datetime)::date = '2015-06-26'::date)

Rows Removed by Filter: 55051918
-> Hash (cost=4292.99..4292.99 rows=134559 width=10) (never executed)
-> Seq Scan on users (cost=0.00..4292.99 rows=134559 width=10) (never executed)
Filter: ((us_category_id)::text <> ''::text)

"Planning time: 1.309 ms"
"Execution time: 899141.789 ms"

Date is changed. How can I speed up the query?

Table and index creating

CREATE TABLE web_hits (
  h_id integer NOT NULL DEFAULT nextval('w_h_seq'::regclass),
  h_user_id character varying,
  h_datetime timestamp without time zone,
  h_db_id character varying,
  h_voc_prefix character varying,
  ...
  h_bot_chek integer, -- 1-бот...
  CONSTRAINT w_h_pk PRIMARY KEY (h_id)
);
ALTER TABLE web_hits OWNER TO postgres;
COMMENT ON COLUMN web_hits.h_bot_chek IS '1-бот, 0-не бот';

CREATE INDEX h_datetime ON web_hits (h_datetime);
CREATE INDEX h_db_index ON web_hits (h_db_id COLLATE pg_catalog."default");
CREATE INDEX h_pref_index ON web_hits (h_voc_prefix COLLATE pg_catalog."default" text_pattern_ops);
CREATE INDEX h_user_index ON web_hits (h_user_id text_pattern_ops);

 CREATE TABLE users (
  us_id character varying NOT NULL,
  us_category_id character varying,
  ...
  CONSTRAINT user_pk PRIMARY KEY (us_id),
  CONSTRAINT cities_users_fk FOREIGN KEY (us_city_home)
      REFERENCES cities (city_id),
  CONSTRAINT countries_users_fk FOREIGN KEY (us_country_home)
      REFERENCES countries (country_id),
  CONSTRAINT organizations_users_fk FOREIGN KEY (us_institution_id)
      REFERENCES organizations (org_id),
  CONSTRAINT specialities_users_fk FOREIGN KEY (us_speciality_id)
      REFERENCES specialities (speciality_id),
  CONSTRAINT us_affiliation FOREIGN KEY (us_org_id)
      REFERENCES organizations (org_id),
  CONSTRAINT us_category FOREIGN KEY (us_category_id)
      REFERENCES categories (cat_id),
  CONSTRAINT us_reading_room FOREIGN KEY (us_reading_room_id)
      REFERENCES reading_rooms (rr_id)
);
ALTER TABLE users OWNER TO sveta;
COMMENT ON COLUMN users.us_type IS '0-аноним, 1-читатель, 2-удаленный';

CREATE INDEX us_cat_index ON users (us_category_id);
CREATE INDEX us_user_index ON users (us_id text_pattern_ops);