Question about Foreign key constraint causes "costly sequential scans"?

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.

As a rule I index foreign keys.

Emi Lu <emilu@cs.concordia.ca> writes:

But I got a warning msg from postgresql as:
foreign key constraint "Aclo3_fk" will require costly sequential scans

Try to make the referencing and referenced columns the same datatype.

regards, tom lane

On Tue, Aug 23, 2005 at 10:30:14AM -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

Is there not a DETAIL message following this warning? It should
explain the problem: the referring and the referred-to columns are
of different types.

Additionally, it's usually a good idea to create an index on the
referring column (A.col3) to speed up referential integrity checks
when modifying the referred-to table (B).

--
Michael Fuhr

Thanks a lot for all helps. I do not have warnings anymore :-)

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