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Re: Join query including two generate_series calls causes big memory growth and crash

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2011/4/20  <tv@xxxxxxxx>:
>> Hello,
>>
>> I'm executing this query:
>>
>> SELECT x, y, another_field FROM generate_series(1, 10) x,
>> generate_series(1, 10) y, my_table
>
> Well, do you realize this is a cartesian product that gives
>
> 10 x 10 x 360000 = 36.000.000
>
> rows in the end. Not sure how wide is the third table (how many columns
> etc.) but this may occupy a lot of memory.
>

Yes, I know it. But I expect memory swapping in this situation, not crashing.


>> The field 'another_field' belongs to 'my_table'. And that table has
>> 360000 entries. In a 64 bits machine, with 4GB RAM, Ubuntu 10.10 and
>> postgres 8.4.7, the query works fine. But in a 32 bits machine, with
>> 1GB RAM, Ubuntu 9.10 and postgres 8.4.7, the query process is killed
>> after taking about 80% of available memory. In the 64 bits machine the
>> query takes about 60-70% of the available memory too, but it ends.
>> And this happens even if I simply get x and y:
>>
>> SELECT x, y FROM generate_series(1, 10) x, generate_series(1, 10) y,
>> my_table
>
> The result is still 36 million rows, so there's not a big difference I guess.
>

Yes, silly example. I only wanted to delete my table's field from equation.


>> Is it normal? I mean, postgres has to deal with millions of rows, ok,
>> but shouldn't it start swapping memory instead of crashing? Is a
>> question of postgres configuration?
>
> I guess that's the OOM killer, killing one of the processes. See this
>
> http://en.wikipedia.org/wiki/Out_of_memory
>
> so it's a matter of the system, not PostgreSQL - the kernel decides
> there's not enough memory, chooses one of the processes and kills it.
> PostgreSQL is a victim in this case.
>
> Tomas
>
>

Ok, I got it. Anyway, my question again: could I expect some help from
postgres backend to avoid this situation? Something like "I don't want
to be killed by the OOM killer because one of my threads. I'll try
this..."

Maybe is my responsibility, changing some configuration parameters,
like the "\set FETCH_COUNT 1000" Tomas Lane has suggested...

Thanks again,

-- 
Jorge Arévalo
Internet & Mobilty Division, DEIMOS
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