I have an events table that records page views and purchases (type = 'viewed' or type='purchased'). I have a query that figures out "people who bought/viewed this also bought/viewed that".
It worked fine, taking about 0.1 seconds to complete, until a few hours ago when it started taking hours to complete. Vacuum/analyze didn't help. Turned out there was one session_id that had 400k rows in the system. Deleting that made the query performant again.
Is there anything I can do to make the query work better in cases like that? Missing index, or better query?
This is on 9.3.5.
explain select
e1.product_id,
e2.site_id,
e2.product_id,
count(nullif(e2.type='viewed', false)) view_count,
count(nullif(e2.type='purchased', false)) purchase_count
from events e1
join events e2 on e1.session_id = e2.session_id and e1.type = e2.type
where
e1.product_id = '82503' and
e1.product_id != e2.product_id
group by e1.product_id, e2.product_id, e2.site_id;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------
GroupAggregate (cost=828395.67..945838.90 rows=22110 width=19)
-> Sort (cost=828395.67..840117.89 rows=4688885 width=19)
Sort Key: e1.product_id, e2.product_id, e2.site_id
-> Nested Loop (cost=11.85..20371.14 rows=4688885 width=19)
-> Bitmap Heap Scan on events e1 (cost=11.29..1404.31 rows=369 width=49)
Recheck Cond: (product_id = '82503'::citext)
-> Bitmap Index Scan on events_product_id_site_id_idx (cost=0.00..11.20 rows=369 width=0)
Index Cond: (product_id = '82503'::citext)
-> Index Scan using events_session_id_type_product_id_idx on events e2 (cost=0.56..51.28 rows=12 width=51)
Index Cond: ((session_id = e1.session_id) AND (type = e1.type))
Filter: (e1.product_id <> product_id)
(11 rows)
recommender_production=> \d events
Table "public.events"
Column | Type | Modifiers
-------------+--------------------------+-----------------------------------------------------
id | bigint | not null default nextval('events_id_seq'::regclass)
user_id | citext |
session_id | citext | not null
product_id | citext | not null
site_id | citext | not null
type | text | not null
happened_at | timestamp with time zone | not null
created_at | timestamp with time zone | not null
Indexes:
"events_pkey" PRIMARY KEY, btree (id)
"events_product_id_site_id_idx" btree (product_id, site_id)
"events_session_id_type_product_id_idx" btree (session_id, type, product_id)
Check constraints:
"events_session_id_check" CHECK (length(session_id::text) < 255)
"events_type_check" CHECK (type = ANY (ARRAY['purchased'::text, 'viewed'::text]))
"events_user_id_check" CHECK (length(user_id::text) < 255)