> 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
    automatically.

> [...]
>
> 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.


Jan

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#======================================================================#
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Tue, 4 Apr 2000 17:00:20 +0200 (CEST)
wieck@debis.com (Jan Wieck)
Re: 7.0 FK trigger question
>

Thanks for the reply. For the time begin I've solved this by copying every
table in the database to a backup table without any constraints, recreating
the tables and copying the data back in. I have to be a bit careful with
doing it all in the right order, although I think I can solve this by doing
everything in a transaction as the constraints are only checked at the end of
transaction?

> >
> > 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.

OK, I'm definitely not being very bright here, but i cannot get my system to
accept the alter column commands. An example on the man pages ,ay help a lot
here! I tried

test=# create table t (i int4);
CREATE
test=# create table t1 (k int4);
CREATE
test=# alter table t1 alter column k add constraint references t(i);
ERROR:  parser: parse error at or near "add"
test=# alter table t1 alter column k constraint references t(i);
ERROR:  parser: parse error at or near "constraint"
test=# alter table t1 alter k constraint references t(i);
ERROR:  parser: parse error at or near "constraint"
test=# alter table t1 alter column k create constraint references t(i);
ERROR:  parser: parse error at or near "create"

So what am I doing wrong?

Thanks,

Adriaan


Wed, 05 Apr 2000 08:31:33 +0300
Adriaan Joubert <a.joubert@albourne.com>
Re: 7.0 FK trigger question
> Thanks for the reply. For the time begin I've solved this by copying every
> table in the database to a backup table without any constraints, recreating
> the tables and copying the data back in. I have to be a bit careful with
> doing it all in the right order, although I think I can solve this by doing
> everything in a transaction as the constraints are only checked at the end of
> transaction?

    By  default,  constraints  are  checked  at end of statement.
    Constraints can be specified DEFERRABLE, then you can do  SET
    CONSTRAINTS  ... DEFERRED which will delay them until COMMIT.

> OK, I'm definitely not being very bright here, but i cannot get my system to
> accept the alter column commands. An example on the man pages ,ay help a lot
> here! I tried
>
> test=# create table t (i int4);
> CREATE
> test=# create table t1 (k int4);
> CREATE
> test=# alter table t1 alter column k add constraint references t(i);
> ERROR:  parser: parse error at or near "add"
> test=# alter table t1 alter column k constraint references t(i);
> ERROR:  parser: parse error at or near "constraint"
> test=# alter table t1 alter k constraint references t(i);
> ERROR:  parser: parse error at or near "constraint"
> test=# alter table t1 alter column k create constraint references t(i);
> ERROR:  parser: parse error at or near "create"
>
> So what am I doing wrong?

    alter table t1 add constraint chk_k foreign key (k) references t (i);

    The referenced column(s) (t.i in your case above) must not be
    a  primary  key  -  any combination is accepted. SQL standard
    requires that  there  is  a  unique  index  defined  for  the
    referenced  columns so it is guaranteed that FKs reference to
    exactly ONE row. Actually Postgres doesn't check or force it,
    so you have to take care yourself.  For example:

    create table t (i integer, j integer);
    create unique index t_pk_idx_1 on t (i, j);    -- DON'T FORGET THIS!
    create table t1 (k integer, l integer,
        foreign key (k, l) references t (i, j));

    BTW: all existing data is checked at ALTER TABLE time.

    And  our  implementation  of  FK is based on SQL3. So you can
    specify match  type  FULL  (PARTIAL  will  be  in  7.1),  and
    referential  actions (ON DELETE CASCADE etc.) too. It is nice
    to define ON UPDATE CASCADE, because if you UPDATE a PK,  all
    referencing FKs will silently follow then.


Jan

--

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



Wed, 5 Apr 2000 13:50:43 +0200 (CEST)
wieck@debis.com (Jan Wieck)
Re: 7.0 FK trigger question