Re: Query performance

On Sat, Jan 24, 2015 at 9:41 PM, Joe Van Dyk <joe(at)tanga(dot)com> wrote:

> 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.
>
> The below is reproduced at the following URL if it's not formatted
> correctly in the email.
> https://gist.githubusercontent.com/joevandyk/cb8f4afdb6c1b178c606/raw/9940bbe033ebd56d38caa46e33c1ddfd9df36eda/gistfile1.txt
>
> 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)
>
>
>
>
After removing the session with 400k events, I was able to do an explain
analyze, here is one of them:
http://explain.depesz.com/s/PFNk