Re: slow query

2010/6/4  <tv@fuzzy.cz>:
>> I am reposting as my original query was mangled
>>
>> The link to the explain plan is here as it does not paste well into
>> the email body.
>>
>> http://explain.depesz.com/s/kHa
>>
>>
>> The machine is a 2 cpu quad core 5430 with 32G RAM and 6x450G 15K
>> single raid-10 array
>>
>> 1G work_mem
>> default_statistics_target=1000
>> random_page_cost=1
>
> Are you sure it's wise to set the work_mem to 1G? Do you really need it?
> Don't forget this is not a 'total' or 'per query' - each query may
> allocate multiple work areas (and occupy multiple GB). But I guess this
> does not cause the original problem.
>
> The last row 'random_page_cost=1' - this basically says that reading data
> by random is just as cheap as reading data sequentially. Which may result
> in poor performance due to bad plans. Why have you set this value?
>
> Sure, there are rare cases where 'random_page_cost=1' is OK.

The default for 8.4 is 2
I tried with 2 and 1..but the results are not very different. I
understand that for fast disks (which we have with a decent Raid 10
setup)..the random_page_cost can be lowered as needed..but I guess it
did not make a difference here.


>
>>
>> I am curious why the hash join takes so long. The main table
>> dev4_act_dy_fact_2010_05_t has 25 million rows. The table is
>> partitioned into 3 parts per month. Remaining tables are very small (
>> < 1000 rows)
>
> Well, the real cause that makes your query slow is the 'index scan' part.
>
> Index Scan using dev4_act_dy_fact_2010_05_t3_thedate on
> dev4_act_dy_fact_2010_05_t3 a (cost=0.00..94041.89 rows=204276 width=60)
> (actual time=164533.725..164533.725 rows=0 loops=1)
>
> The first thing to note here is the difference in expected and actual
> number of rows - the planner expects 204276 but gets 0 rows. How large is
> this partition?

The partition has 25 million rows with indexes on theDate, node_id..
I altered the random_page_cost to 4 (1 more than the default)..still
slow. These tables are analyzed every day
I have an index on each field used in the where criteria,
>
> Try to analyze it, set the random_page_cost to something reasonable (e.g.
> 4) and try to run the query again.
>
> Tomas
>
>