unique constraints on foreign keys

[Jaime Casanova]

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

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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match

[Stephan Szabo]

Post by Jaime Casanova
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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[Ramiro Batista da Luz]

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[Jaime Casanova]

Post by Stephan Szabo

Post by Jaime Casanova
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.
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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match

[Stephan Szabo]

Post by Jaime Casanova

Post by Stephan Szabo

Post by Jaime Casanova
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.

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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match