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

Scott Marlowe wrote:

>> 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".

But this doesn't really match my question - I wanted to know whether
checking an FK on =one= column would use a composite key on =several=
columns. Modifying your example:

> create table a (i int PRIMARY KEY, j int, info text);
> create table b (o int REFERENCES A, p int, moreinfo text, PRIMARY
> KEY (O,P));
> insert into a values (1,2,'abc');
> insert into b values (1,2,'def');
>
> delete from a where i = 1;

Here, the FK is a simple one, and the referential integrity machinery
simply needs to check whether there is a row in table B with O=1. My
question is whether it will use the composite PK index.

I guess a generalization of my question is whether the FK-checking
machinery simply does a SELECT against the referencing column. That
is, in this example, if the following effectively happens:

SELECT * FROM B WHERE O = 1;

then PG will use whatever index might make the query faster. Is this
in fact the case, that I should think of the FK machinery as simply
doing the appropriate SELECT?

Thanks.

- John D. Burger
MITRE