From: | John Burger <john(at)mitre(dot)org> |
---|---|
To: | PostgreSQL General <pgsql-general(at)postgresql(dot)org> |
Cc: | Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> |
Subject: | Re: Will PG use composite index to enforce foreign keys? |
Date: | 2007-12-03 14:08:49 |
Message-ID: | 29DBBD3D-720F-473E-929E-6632D0DD0BD2@mitre.org |
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Lists: | pgsql-general |
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
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