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To: mark.kirkwood@catalyst.net.nz,
 "Mario Splivalo" <mario.splivalo@megafon.hr>
Cc: pgsql-performance@postgresql.org
Subject: Re: SELECT INTO large FKyed table is slow
References: <4CF24103.8030209@megafon.hr> <4CF3522B.2050203@catalyst.net.nz>
	<4CF39CF4.2050506@megafon.hr>
Date: Mon, 29 Nov 2010 17:53:19 +0100
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From: "Pierre C" <lists@peufeu.com>
Message-ID: <op.vmxyi50oeorkce@apollo13>
In-Reply-To: <4CF39CF4.2050506@megafon.hr>
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> Yes, since (sample_id, drone_id) is primary key, postgres created  
> composite index on those columns. Are you suggesting I add two more  
> indexes, one for drone_id and one for sample_id?

(sample_id,drone_id) covers sample_id but if you make searches on drone_id  
alone it is likely to be very slow since you got a large number of  
sample_ids. Postgres can use any column of a multicolumn index but it is  
only interesting performance-wise if the cardinality of the first  
(ignored) columns is low. If you often make searches on drone_id, create  
an index. But this isn't what is slowing your foreign key checks.

>> Also it would be worthwhile for you to post the output of:
>>
>> EXPLAIN ANALYZE INSERT INTO drones_history (sample_id, drone_id,
>> drone_log_notice, drone_temperature, drone_pressure)
>> SELECT * FROM tmpUpdate;
>>
>> to the list, so we can see what is taking the time.
>
> Is there a way to do so inside plpgsql function?
>
> I can recreate the whole process within psql and then post the explain  
> analyze, it would just take me some time to do so. I'll post as soon as  
> I'm done.

Yes, this would be interesting.