Re: using a sequence as the functional equivalent to Oracle

[Oisin Glynn <me@xxxxxxxxxxxxxx>]

Wm.A.Stafford wrote:
> I'm trying to use a temporary sequence to duplicate the functionality 
> of the Oracle rownum pseudo-column
> as suggested by Scott Marlow in the archives: 
> http://archives.postgresql.org/pgsql-sql/2005-05/msg00126.php.
>
> The Oracle based application I'm porting to PostgreSQL used  rownum to 
> select the 'next' block of rows to
> process by specifying a where clause with something like " where 
> rownum>x and rownum<y "
>
> My basic PostgreSQL query is:
>
> drop sequence rownum ;
> create temp sequence rownum;
>
> select B.rownum , B.id from
> (select nextval('rownum') as rownum, A.*  from
> (select distinct id  from ... where ... order by ... DESC
> ) as A
> ) as B
> where id>0
>
> This basic query produces the following result set:
> rownum     id
> --------+---------
>      1       10038
>      2       10809
>      3       10810
>      4       22549
>      5       23023
>
> However, if I add a where clause referencing rownum for example: where 
> id>0 and rownum>0
> I get the following:
>
> rownum   id
> -------+---------
>  11        10038
>  12        10809
>  13        10810
>  14        22549
>  15        23023
>
> It appears as if rownum has been incremented as a result of three 
> passes over the five row result set.
>
> Can someone explain what is going on?  And more to to point, if this 
> is expected behavior, is there a standard PostgreSQL way to select a 
> 'block' of rows from a result set based on row number?
>
> Thanks,
> -=bill
>
>
>
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I have done this using limit and offset like the following
select * from foo order by bar limit 10 offset 50;--giving the 10 rows 
from position 51 onwards (offset is zero based)

Oisin

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