Re: [HACKERS] LIMIT and JOINS

[Tom Lane]

"Gregor Zeitlinger" <[EMAIL PROTECTED]> writes:
> select * from T1 join T2 on T1.id =3D T2.id LIMIT 1
>
> Conceptually, it should be possible to fetch one row from T1 and T2, =
> i.e. to propagate the LIMIT clause.
>
> I was wondering what the exact requirements are to propagate the LIMIT =
> clause.

Indexes would help ...

regards, tom lane

---(end of broadcast)---
TIP 2: Don't 'kill -9' the postmaster

[HACKERS] LIMIT and JOINS

[Gregor Zeitlinger]

Hi,

 

I have a question on implementation
of the LIMIT SQL clause.

 

Using the LIMIT clause, I want
to reduce the computation time for a query.

 

When I try to limit the result
of a joined table, however, the join will be computed first (which takes very long).

 

Lets suppose the following example:

 

select * from T1 join T2 on T1.id = T2.id LIMIT 1

 

Conceptually, it should be possible
to fetch one row from T1 and T2, i.e. to propagate the LIMIT clause.

 

I was wondering what the
exact requirements are to propagate the LIMIT clause.

 

Do I need a foreign key
relation between T1 and T2?

Do I need to use a full outer
join in order to propagate the LIMIT clause?

 

Thanks

 

Gregor
 Zeitlinger

LUCAS Product Development

 

Torex Retail Solutions GmbH

 

Schwedenstr. 9,
D-13359 Berlin 

Tel. +49 (0) 30
49901-243

Fax +49 (0) 30
49901-139

 

Mailto:[EMAIL PROTECTED]

http://www.torexretail.de