Simon Riggs [EMAIL PROTECTED] writes:
On Fri, 2008-02-01 at 19:25 +0100, [EMAIL PROTECTED] wrote:
PostgreSQL allows backward reading tuples writing the tuple's length
after and before the tuple proper, in case a 'randomAccess' is
requested.
Is there any example of backward reading tuples into PostgreSQL code?
Don't think so, but we don't always use randomAccess anyway. Sounds like
we might be able to drop the length at the end of each tuple in those
cases...
We already do. We only generate the frozen tape when we think it might be
necessary.
I think the easiest (possibly only?) way to trigger this case is to run the
query in a cursor like:
postgres=# set enable_indexscan = off;
SET
postgres=# explain select * from h order by i;
QUERY PLAN
Sort (cost=61772.22..62022.20 rows=4 width=512)
Sort Key: i
- Seq Scan on h (cost=0.00..7666.94 rows=4 width=512)
(3 rows)
postgres=# begin;
BEGIN
postgres=# declare c cursor for select * from h order by i;
DECLARE CURSOR
postgres=# fetch 5 from c;
i | r
---+--
1 | 10352
2 | 15034
3 | 91904
4 | 89058
5 | 87001
(5 rows)
postgres=# fetch backward 5 from c;
i | r
---+--
4 | 89058
3 | 91904
2 | 15034
1 | 10352
(4 rows)
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's RemoteDBA services!
---(end of broadcast)---
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
