Re: Will PG use composite index to enforce foreign keys?

On Nov 29, 2007 10:51 AM, John Burger <john@mitre.org> wrote:
> Hi -
>
> I know that the foreign key machinery will use an index on the
> referring column if one exists.  My question is whether it will use a
> composite index?  For instance:
>
> create table allLemmaSenseMap (
>    wordID     integer references allLemmas,
>    senseID    integer references allSenses,
>    primary key (wordID, senseID)
> );
>
> If I delete something from allLemmas, will the FK check use the PK
> index above?  (I know I should at least have an index on senseID as
> well, because of the other foreign key.)

Yes.  It will

> As a secondary question, is there any way I could have answered this
> myself, using analyze, the system catalogs, etc?  ANALYZE DELETE
> doesn't seem to show the FK checking that must go on behind the scenes.

You could have coded up an example to see if it worked I guess.
Here's a short example:

create table a (i int, j int, info text, primary key (i,j));
create table b (o int, p int, moreinfo text, foreign key (o,p) references a);
insert into a values (1,2,'abc');
insert into b values (1,2,'def');
INSERT 0 1
insert into b values (1,3,'def');
ERROR:  insert or update on table "b" violates foreign key constraint "b_o_fkey"
DETAIL:  Key (o,p)=(1,3) is not present in table "a".
delete from a;
ERROR:  update or delete on table "a" violates foreign key constraint
"b_o_fkey" on table "b"
DETAIL:  Key (i,j)=(1,2) is still referenced from table "b".