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Re: 7.0 FK trigger question

From: wieck(at)debis(dot)com (Jan Wieck)
To: Adriaan Joubert <a(dot)joubert(at)albourne(dot)com>
Cc: Postgresql <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: 7.0 FK trigger question
Date: 2000-04-04 15:00:20
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
> Hi,
>     due to the limitations in alter table, I generate some SQL to
> implement changes to tables. This works along the lines of
> 1. drop fk triggers on old table
> 2. rename serial sequences on old table
> 3. drop indexes on old table
> 4. rename old table
> 5. create new table
> 6. insert into new table select ... from old table
> 7. drop new sequences/rename old sequences
> 8. recreate fk triggers

  9. drop old table

    If   you   do   9.  you  can  skip  1.  because  that's  done

> [...]
> This looks kind-of hairy to drop and recreate correctly.
> I thought an alternative may be to change the oid's in pg_trigger. But I
> saw that the oid's of the tables are part of the trigger name. I could
> probably recreate the trigger names with different oid's but this looks
> like asking for trouble.

    Not exactly. The OIDs in the trigger names are just ones that
    CREATE  CONSTRAINT  TRIGGER  allocates  itself to give any of
    them a unique name. They aren't used anywhere else, so  don't
    care.  And  BTW: specifying a constraint really invokes these
    commands internally.

> So what is the best solution? It would be great if there would be some
> way to drop foreign key triggers and re-instate them. This would also
> help with loading data where there are circular dependencies of foreign
> keys, as one could drop a trigger to break the loop, load the data, and
> re-instate the triggers.

    Ideally you would use correct ALTER TABLE ... ADD  CONSTRAINT
    commands, which are implemented in 7.0.

    pg_dump  actually does sort of this "disable RI triggers" for
    data only dumps. You  might  want  to  setup  a  simple  test
    database and take a data only dump to see the mechanism.

> So I guess my question really boils down to: is it possible to write a
> function that drops a foreign key trigger or re-instates it? This should
> really be ALTER TABLE  table ALTER COLUMN column (DROP|CREATE)
> CONSTRAINT.... or something along those lines.

    There's   still   something  missing  in  ALTER  TABLE.  DROP
    CONSTRAINT is one of  them,  but  since  your  sequencs  with
    renaming  the old etc. is the safest possibility anyway, it's
    not that high priority.



# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#========================================= wieck(at)debis(dot)com (Jan Wieck) #


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