Postgres: How to JOIN - postgresql

Question

I am working on Postgres 9.3. I have two tables, the first for payment items:
Table "public.prescription"
Column | Type | Modifiers
-------------------+-------------------------+--------------------------------------------------------------------
id | integer | not null default nextval('frontend_prescription_id_seq'::regclass)
presentation_code | character varying(15) | not null
presentation_name | character varying(1000) | not null
actual_cost | double precision | not null
pct_id | character varying(3) | not null
And the second for organisations:
Table "public.pct"
Column | Type | Modifiers
-------------------+-------------------------+-----------
code | character varying(3) | not null
name | character varying(200) |
I have a query to get all the payments for a particular code:
SELECT sum(actual_cost) as total_cost, pct_id as row_id
FROM prescription
WHERE presentation_code='1234' GROUP BY pct_id
Here is the query plan for that query.
Now, I'd like to annotate each row with the name property of the associated organisation. This is what I'm trying:
SELECT sum(prescription.actual_cost) as total_cost, prescription.pct_id, pct.name as row_id
FROM prescription, pct
WHERE prescription.presentation_code='0212000AAAAAAAA'
GROUP BY prescription.pct_id, pct.name;
Here's the ANALYSE for that query. It's incredibly slow: what am I doing wrong?
I think there must be a way to annotate each row with the pct.name AFTER the first query has run, which would be faster.

Answer 1

With JOIN (LEFT JOIN in this case, because we want the line even if there is no pct):
SELECT
sum(prescription.actual_cost) as total_cost,
prescription.pct_id,
pct.name as row_id
FROM prescription
LEFT JOIN pct ON pct.code = prescription.pct_id
WHERE
prescription.presentation_code='0212000AAAAAAAA'
GROUP BY
prescription.pct_id,
pct.name;
I don't know if it's work well, I didn't try this query.

Answer 2

You are taking data from 2 tables, but you do not join the tables in any way. Effectively, you make a full join, resulting in the Cartesian product of both tables. If you look at your ANALYZE statistics, you see that your nested loop processes 62 million rows. that takes time.
Add in a join condition to make this all fast:
SELECT sum(prescription.actual_cost) as total_cost, prescription.pct_id, pct.name as row_id
FROM prescription
JOIN pct On pct.code = prescription.pct_id
WHERE prescription.presentation_code = '0212000AAAAAAAA'
GROUP BY prescription.pct_id, pct.name;