Re: Question about Foreign key constraint causes "costly

On Tue, Aug 23, 2005 at 02:53:33PM +0000, Matt Miller wrote:
> On Tue, 2005-08-23 at 10:30 -0400, Emi Lu wrote:
> > I'd like to setup foreign key constraint for A.col3, as the following:
> > CONSTRAINT Aclo3_fk FOREIGN KEY (col3) REFERENCES B(colB1)
> >
> > But I got a warning msg from postgresql as:
> >
> >    foreign key constraint "Aclo3_fk" will require costly sequential
> > scans
>
> Index the foreign key:
>
>     create index a_col3_ix on a (col3);
>
> and then drop an recreate the foreign key and see if the warning is
> still there.

That's a good idea, but it's not the condition that elicits the "will
require costly sequential scans" warning.  That warning is caused by
the keys being of different types (see ATAddForeignKeyConstraint()
in src/backend/commands/tablecmds.c in the source code for 8.0 and
later).

--
Michael Fuhr