Re: [SQL] Move forward all (Was Re: How to get last 10 rows in a table on a large database?)

Stoyan Genov wrote:

> > > > > You are unable to say:
> > > > >         begin transaction;
> > > > >                 declare tbl_cur cursor for .... .....
> > > > >                 move forward all in tbl_cur;
> > > > >                 fetch backward 10 in tbl_cur;
> > > > >                 ...........
> > > > >         end transaction;
> > > > > because when you "move forward all" the result gets lost.
> > > >
> > > > Huh?  It seems to work fine for me.
> > >
> > > Well, it does not seem to work for me neither on the 6.4.2 nor on the
> > > 6.5.(1|2)
> > > version. I'll try 6.5.3 as well...
> > > Any ideas why this is so?
> > >
> >
> > Doesn't your query have any qualification(WHERE clause) about
> > columns in index ?
> > If there's no qualification,PostgreSQL optimizer chooses sequential
> > scan(however current developing tree probably chooses Index scan
> > in case of ORDER BY).
> > In case of Index scan,"fetch backward .." after "move forward all" is
> > possible after 6.5 ,
> > But in case of sequential scan,maybe it's still impossible.
>
> I have tested this - for a sequental scan it is still impossible (talking
> 6.4.2 and 6.5.* versions)
>
> > I knew the way to fix it but am not sure now.
> > Do you really want to make it possible ?
> > It isn't an appropriate way to get last rows because "move forward
> > all" takes very long time.
> >
>
> Of course it is not an appropriate way to get the last rows like this.
> The "right" way IMHO is to reverse the order in the query and to get the
> first
> rows.
>
> It is just for the sake of truth...
>

OK Attached is a patch.
Could you apply to 6.5.3 ?

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp




*** access/heap/heapam.c.orig    Mon Aug  2 14:56:36 1999
--- access/heap/heapam.c    Tue Nov  9 09:06:58 1999
***************
*** 833,842 ****
                  ReleaseBuffer(scan->rs_pbuf);
              scan->rs_ptup.t_data = NULL;
              scan->rs_pbuf = InvalidBuffer;
-             if (BufferIsValid(scan->rs_nbuf))
-                 ReleaseBuffer(scan->rs_nbuf);
-             scan->rs_ntup.t_data = NULL;
-             scan->rs_nbuf = InvalidBuffer;
              return NULL;
          }

--- 833,838 ----
***************
*** 915,924 ****
                  ReleaseBuffer(scan->rs_nbuf);
              scan->rs_ntup.t_data = NULL;
              scan->rs_nbuf = InvalidBuffer;
-             if (BufferIsValid(scan->rs_pbuf))
-                 ReleaseBuffer(scan->rs_pbuf);
-             scan->rs_ptup.t_data = NULL;
-             scan->rs_pbuf = InvalidBuffer;
              HEAPDEBUG_6;        /* heap_getnext returning EOS */
              return NULL;
          }
--- 911,916 ----