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

On Nov 29, 2007 10:51 AM, John Burger <john(at)mitre(dot)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".