Subject: Re: faster INSERT with possible pre-existing row?
From: Sven Willenberger <sven@dmv.com>
To: Dan Harris <fbsd@drivefaster.net>
Cc: pgsql-performance@postgresql.org
In-Reply-To: <AB60A4E4-1DF1-459E-8866-007810D68419@drivefaster.net>
References: <AB60A4E4-1DF1-459E-8866-007810D68419@drivefaster.net>
Content-Type: text/plain
Date: Tue, 26 Jul 2005 15:03:17 -0400
Message-Id: <1122404597.1605.50.camel@lanshark.dmv.com>
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On Tue, 2005-07-26 at 10:50 -0600, Dan Harris wrote:
> I am working on a process that will be inserting tens of million rows  
> and need this to be as quick as possible.
> 
> The catch is that for each row I could potentially insert, I need to  
> look and see if the relationship is already there  to prevent  
> multiple entries.  Currently I am doing a SELECT before doing the  
> INSERT, but I recognize the speed penalty in doing to operations.  I  
> wonder if there is some way I can say "insert this record, only if it  
> doesn't exist already".  To see if it exists, I would need to compare  
> 3 fields instead of just enforcing a primary key.
> 
> Even if this could be a small increase per record, even a few percent  
> faster compounded over the whole load could be a significant reduction.
> 
> Thanks for any ideas you might have.
> 

Perhaps a trigger:

CREATE FUNCTION verify_unique() RETURNS TRIGGER AS $func$
BEGIN
	PERFORM a,b,c FROM table1 WHERE a = NEW.a and b = NEW.b and c = NEW.c;
	IF FOUND THEN 
		RETURN NULL;
	END IF;
	RETURN NEW;
END;
$func$ LANGUAGE plpgsql STABLE;

CREATE TRIGGER verify_unique BEFORE INSERT ON table1 FOR EACH ROW
EXECUTE PROCEDURE verify_unique();

Triggers are fired on COPY commands and if table1 is able to be cached
and you have an index on table1(a,b,c) the results should be fairly
decent. I would be interested in seeing the difference in timing between
this approach and the temp table approach.