Re: Optimize SELECT * from table WHERE foreign_key_id IN (key1,key2,key3,key4...)

On 03/06/2013 00:51, Niels Kristian Schjødt wrote:
> Hi, thanks for answering. See comments inline.
>
> Den 05/03/2013 kl. 15.26 skrev Julien Cigar <jcigar@ulb.ac.be>:
>
>> On 03/05/2013 15:00, Niels Kristian Schjødt wrote:
>>> Hi,
>>>
>>> I'm running a rails app, where I have a model called Car that has_many Images. Now when I tell rails to include
thoseimages, when querying say 50 cars, then it often decides to use a SELECT * from images WHERE car_id IN
(id1,id2,id3,id4…)instead of doing a join. 
>> why do you want a join here ? if you don't need any "cars" data there is no need to JOIN that table.
> I need both
>> Now a select ... from ... where id in (id1, id2, ..., idn) isn't very scalable.
>>
>> Instead of passing id1, id2, ..., idn you'be better pass the condition and do a where id in (select ... ), or where
exists(select 1 ... where ...), or a join, or … 
>>
> I tried this now, and it doesn't seem to do a very big difference unfortunately…

could you paste the full query, an explain analyze of it, and some
details about your config (how much ram ? what's your: shared_buffers,
effective_cache_size, cpu_tuple_cost, work_mem, ...) ?

>>> Now either way it uses the index I
>>> have on car_id:
>>>
>>> Index Scan using car_id_ix on adverts  (cost=0.47..5665.34 rows=1224 width=234)
>>>     Index Cond: (car_id = ANY ('{7097561,7253541,5159633,6674471,...}'::integer[]))
>>>
>>> But it's slow, it's very slow. In this case it took 3,323ms
>> 3ms isn't slow
>>
> Sorry, it's 3323ms!
>
>>> Can I do anything to optimize that query or maybe the index or something?
>> your index is already used
> Okay this leaves me with - "get better hardware" or?
>
>>> The table has 16.000.000 rows
>>>
>>
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However, many electrons were terribly inconvenienced.