Обсуждение: unique constraints on foreign keys
Hi all, I have a reference table that holds all the status used in a system (it's a very little table just 10 or 15 rows). create table status ( cod_status char(2) not null primary key, nam_status text not null ); create table some_other_table ( ... definition of the table ... status char(2) not null references status ); ok. This is the idea if a want to do a reference to the status table i need to create a primary key or a unique index on the status table. Obviously the planner will choose always a seq scan in such a table (it's obvious to you, it's obvious to me, but not to the planner), but the planner will calculate the cost of use the index and will choose the seq. So, there is a way to teach the planner always do a seq on that table and do not ask for use the index? There is a way to not enforce the creation of a primary or unique index on a referenced table? (I mean, a way that not involves the use of a "set" instruction because the odbc not allow such instructions) Thanx in advance, Jaime Casanova _________________________________________________________ Do You Yahoo!? Información de Estados Unidos y América Latina, en Yahoo! Noticias. Visítanos en http://noticias.espanol.yahoo.com
On Mon, 20 Sep 2004, Jaime Casanova wrote: > I have a reference table that holds all the status > used in a system (it's a very little table just 10 or > 15 rows). > > create table status ( > cod_status char(2) not null primary key, > nam_status text not null > ); > > create table some_other_table ( > ... definition of the table ... > status char(2) not null references status > ); > > ok. This is the idea if a want to do a reference to > the status table i need to create a primary key or a > unique index on the status table. > > Obviously the planner will choose always a seq scan in > such a table (it's obvious to you, it's obvious to me, > but not to the planner), but the planner will > calculate the cost of use the index and will choose > the seq. > So, there is a way to teach the planner always do a > seq on that table and do not ask for use the index? > There is a way to not enforce the creation of a > primary or unique index on a referenced table? Not really. It's not the index that's important per-se, it's the uniqueness that is implied. The SQL spec requires the set of referenced columns to be the set of columns of a unique or primary key on the referenced table and we enforce that. Without that the behavior of all the foreign key types becomes as complicated as the match partial behavior because you have to be able to deal with multiple matching referenced rows.
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--- Stephan Szabo <sszabo@megazone.bigpanda.com> escribió: > > On Mon, 20 Sep 2004, Jaime Casanova wrote: > > > There is a way to not enforce the creation of a > > primary or unique index on a referenced table? > > Not really. It's not the index that's important > per-se, it's the uniqueness that is implied. OK. What about to teach the planner always do a seq on that table and do not ask for use the index? but not using "set" instructions. can i do that? _________________________________________________________ Do You Yahoo!? Información de Estados Unidos y América Latina, en Yahoo! Noticias. Visítanos en http://noticias.espanol.yahoo.com
On Mon, 20 Sep 2004, Jaime Casanova wrote: > --- Stephan Szabo <sszabo@megazone.bigpanda.com> > escribi�: > > > > On Mon, 20 Sep 2004, Jaime Casanova wrote: > > > > > There is a way to not enforce the creation of a > > > primary or unique index on a referenced table? > > > > Not really. It's not the index that's important > > per-se, it's the uniqueness that is implied. > > OK. What about to teach the planner always do a > seq on that table and do not ask for use the > index? but not using "set" instructions. can i do > that? Not really. Are you worried about the cost of doing the plan analysis for the index scan or that it might choose to actually use the index? Theoretically, it's only going to currently plan the fk scan on status once per connection because that plan should be saved.