Re: Query performance

2015-01-25 7:38 GMT+01:00 Joe Van Dyk <joe(at)tanga(dot)com>:

>
>
> On Sat, Jan 24, 2015 at 10:12 PM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
> wrote:
>
>> Hi
>>
>> this plan looks well
>>
>> Regards
>>
>> Pavel
>>
>
> Here's one that's not quite as well: http://explain.depesz.com/s/SgT
>

I see a possible issue

(product_id <> '81716'::citext) .. this operation is CPU expensive and
maybe nonsense

product_id should be integer -- and if it isn't - it should not be on 4M
rows extremly fast - mainly on citext

try to force a opposite cast - you will safe a case insensitive text
comparation

product_id::int <> 81716

Regards

Pavel

>
> Joe
>
>
>>
>> 2015-01-25 6:45 GMT+01:00 Joe Van Dyk <joe(at)tanga(dot)com>:
>>
>>> Oops, didn't run vacuum analyze after deleting the events. Here is
>>> another 'explain analyze': http://explain.depesz.com/s/AviN
>>>
>>> On Sat, Jan 24, 2015 at 9:43 PM, Joe Van Dyk <joe(at)tanga(dot)com> wrote:
>>>
>>>> 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
>>>>
>>>
>>>
>>
>