Re: insert fail gracefully if primary key already exists

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От Bartosz Dmytrak
Тема Re: insert fail gracefully if primary key already exists
Дата
Msg-id CAD8_UcYgTcg7ZTjuOzt8hbFGLjFgrbHpCAW6aU=CprC7sUD6fw@mail.gmail.com
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Ответ на Re: insert fail gracefully if primary key already exists  (Alessandro Gagliardi <alessandro@path.com>)
Ответы Re: insert fail gracefully if primary key already exists  (Alessandro Gagliardi <alessandro@path.com>)
Список pgsql-novice
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You can user few different solutions:

1. You can perform insert using stored procedure. Inside this procedure You can use BEGIN...EXCEPTION...END construct - this will handle duplicate key error. You can find this useful: http://www.postgresql.org/docs/9.1/static/plpgsql-control-structures.html - there is example of such approach.

2. You can also check if such row exists before insert (also in stored procedure):
CREATE OR REPLACE FUNCTION "myschema"."InsertIfNotExists" (IN "vKeyValue" int, IN "vValue1" text, IN "vValue2" text)
RETURNS VOID
AS
$BODY$
BEGIN
     IF NOT EXISTS (SELECT 1 FROM "myschema".mytable WHERE "MyKey" = "vKeyValue") THEN
         INSERT INTO "myschema".mytable ("MyKey", "Value1", "Value2")
         VALUES ("vKeyValue", "vValue1", "vValue2");

    END IF;
END;
$BODY$ LANGUAGE plpgsql 

3. You can create trigger before insert which will check if record already exists:
trigger function could look like this:
CREATE OR REPLACE FUNCTION "myschema"."checkTriggerFunction" ()
RETURNS TRIGGER
AS
$BODY$
BEGIN
IF EXISTS (SELECT 1 FROM "myschema".mytable WHERE "MyKey" = NEW."MyKey") THEN
RETURN NULL;
ELSE
RETURN NEW;
END IF;
END;
$BODY$
LANGUAGE plpgsql;

and trigger:
CREATE TRIGGER "checkTrigger"
  BEFORE INSERT
  ON "myschema".mytable
  FOR EACH ROW
  EXECUTE PROCEDURE "myschema"."checkTriggerFunction"();


Another question is: do You want to update record if primary key exists or simply ignore insert?
It should be noticed, every check impacts performance.

Regards,
Bartek


2012/2/13 Alessandro Gagliardi <alessandro@path.com>
Sorry I wasn't clear. The application is written in Python. It services requests that may go to an external API or to our production database (which is Mongo). I should add that we have a somewhat unusual situation in which we have two parallel databases: MongoDB for OLTP and newer PostgreSQL for OLAP. Because Postgres is only consuming, it would waste precious cycles to have the client check to see if a record already exists in Postgres before trying to insert it. I'd rather let Postgres deal with that (which it does well enough anyway with my primary key constraints). My Postgres instance is hosted by Heroku and they provide me with a log which is currently being spammed by these "duplicate key value violates unique constraint" errors making it impossible to see if there are other errors I need to be paying more attention to. 


On Sat, Feb 11, 2012 at 12:08 AM, Bartosz Dmytrak <bdmytrak@eranet.pl> wrote:
Could You provide more details?
PostgreSQL does not allow cross database queries.


Regards,
Bartek


2012/2/10 Alessandro Gagliardi <alessandro@path.com>
I have a situation where I am gradually filling in a table from another database as the need requires. As it currently is, whenever a record comes up, it tries to insert it into my database. If the row is already there, the primary key collides and the insert fails. And that's fine except that it makes for a really cluttered log (making it hard to find errors that I really do need to pay attention to). I'm wondering if there's a better (i.e. cleaner, but possibly also more efficient) way to do this. 

Thoughts?

Thank you in advance,
-Alessandro



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